I have a serious Art Historian answer for this! Basically, it's because a lot of early paleoartists were primarily drawing from existing geologic images to imagine deep time. I work mainly with British and US American sources, so that is what I'm most familiar with, but people all over the place got interested in geology once the concept of geologic time caught on. Geology started to come into vogue and emerge as a distinct field in the late eighteenth century, and this resulted in the development of a new type of landscape painting, what Rebeca Bedell (an important art historian working in the area) calls geologic landscapes. Basically, these are landscape paintings that pay special attention to depicting rocks and geologic processes like erosion, sedimentation, glaciation, earthquakes and for our interests, volcanism. Geologic landscape is a deliberately broad term covering everything from Thomas Cole's 1836 The Oxbow to John Martin's 1851-53 Great Day of His Wrath
Many early geologists were inspired by Alexander von Humboldt's work from his 1799-1804 expedition to South America and got to surveying the areas where they lived, leading to plenty of fossil discoveries along the way. Most geologic landscapes are of ordinary cliffs and mountains and rivers in Europe. However, von Humboldt's work remained highly influential, particularly his 1805 Naturgemälde, a diagram of Chimborazo and Cotopaxi, inactive and active volcanoes in Ecuador.
Plenty of artists and geologists put a little nod to von Humboldt in their work by including a volcano that looks an awful lot like that smoking top in the background of their paintings. Also, several notable volcanic eruptions happened in the early nineteenth century, most notably a volcano in Europe. Vesuvius/Mt Etna erupted in 1822 and six more times before the century closed out. So you have a situation where geologists were already interested in painting volcanoes, and then they got a whole slew of first-hand accounts of one erupting along with renewed public interest in them. Volcanes started popping up in all different kinds of places from the Paris salon's most lauded history paintings to shitty cartoons in newspapers. To that point, volcanoes became a kind of visual shorthand to indicate an active, changing earth. They showed up in art both from genuine scientific interest and because natural disasters are frightening and therefore draw attention.
Volcanoes and stripey sedimentary rock were simple ways to reference the idea of studying the earth, so lots of artists used them. Paleoartists looking for ways to reference the process of fossilization as well as the idea of geologic time drew on that existing visual language of geologic landscapes. At first, paleoartists were more likely to show a big storm in a seascape for many reasons including references to the biblical flood as well as since so many early big finds were aquatic animals. But volcanoes still cropped up pretty regularly. One shows up in Henry De la Beche's cartoon Awful Changes from 1830. (De la Beche was a geologist and paleontologist) Some anxiety about extinction mixing with the volcano there.
The thing to keep in mind with this is that in many cases, especially with early images, the artist and the scientist were the same person. It wasn't that artists were ignoring scientific accuracy for the sake of a picture, it was that they were trying to present the processes they were studying. Even in many cases where a lay artist was hired to illustrate a book, they worked closely with the scientist to ensure accuracy, or they lost that commission.
Paleoart scenes and more broadly pictorial restorations were mostly made for publication alongside books are articles for a lay audience rather than for scientific journals. Images are expensive to print, especially something as complex as a scene with multiple figures and a fully realized landscape, so authors and publishers had to make decisions about what to show. You get a frontispiece and maybe three to six more full-page illustrations if you're lucky, so you gotta make them good, you gotta cram as much as possible into each image, you gotta make the pictures a little spectacular.
Action, storms with lighting and huge waves, big strange-looking creatures, and volcanoes all make for a nice spectacle to go along with an educational text.
So you get to the late nineteenth/early twentieth century and Mesozoic animals, more specifically dinosaurs have a bit of a boost in popularity. Museums, natural history magazines, book publishers all started hiring professional artists to create restorations of dinosaurs. Restrictions to printing images still apply, plus there's a whole catalogue of around a century's worth of paleoart to look back on for inspiration. Artists like Charles Knight were looking at existing paleoart as well as fossil sources to flesh out their paintings, which meant they saw the volcanoes in paleoart as well as in landscape painting.
There's a volcano in the background of Charles Knight's 1897 Leaping Laelaps
and Several in Rudolph Zallinger's 1947 Age of Reptiles Mural in the Yale Peabody Natural History Museum (I could not find an hq image of this one)
Volcanoes were already an established part of depicting geology by the time paleoart became a thing in the early-mid nineteenth century, and since paleontology was not at all distinct from geology at the time, they naturally became part of paleoart as well. They were already part of the established iconography of paleoart by the time the twentieth century came around to the point that volcanoes got picked up as part of the image of the Mesozoic by artists who weren't at all interested in education, but in entertainment. And volcanoes are a spectacle.
TL;DR: Volcanoes started appearing in landscape paintings depicting geologic processes as a result of scientific interest in geology. This carried over into paleoart, along with depictions of sedimentation and the like and stayed around because natural disasters are scary and interesting and hold people's attention. Dinosaurs and volcanoes became linked because scientists both wanted to demonstrate their knowledge of their field and convince the public their field was worth knowing about. Artists who were/are hired to make an attention-grabbing picture rather than a scientifically rigorous one pick up on the more bombastic parts of the established norms of paleoart and here we are.
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