Loki is literally a spoiled brat with the mind of a frat boy who’s had it good his whole life and gives his adopted Dad shit for nothing. Why are people not realizing this?
I will agree that Loki can be spoiled.
I'm not sure what you mean by "frat boy"? From the movies we cannot tell if he's into partying the same way as Thor, though it does appear that he, along with many other characters of whom we know of from other movies, enjoy a good party if you look at What If's alternate universe.
As far as "adopted Dad," let's take a look here- is stealing a child truly adopting? There is a dark history of human beings doing similarly (especially if you read it as a parallel to those who stole Native and POC children from their parents after massacring, enslaving, or imprisoning their parents and then raised them in a house that is abusive or dismissive of their needs, or literally raised them to be servants to their own biological children), and I would argue that this is more the case than anything.
From what we see, Odin keeps Loki around similarly to how Mother Goethel keeps Rapunzel around- he's a tool that Odin expects to use later on. He even admits this during the scene in the treasure vault where Loki asks him about why his skin is turning blue. Just because people like Odin or Mother Goethel feed and clothe and generally take care of the physical needs of their wards does not necessarily make them good parents.
I would also say that it's pretty shitty for Odin to constantly tell both Thor and Loki that they were both "born to be king." Not only does this set up a rivalry, but it is dishonest as we know that Hela is actually his eldest child and as soon as Odin dies, Hela appears to kick their asses.
Loki also doesn't really directly give Odin shit. He disagrees, but Odin basically silences him every time. When he talks to Odin about his Jotun form, he is hurt and scared about what is happening to him because it's been hidden all his life. Odin's response is to pass out and even THEN, Loki comes up with an underhanded plan to kill the Jotun king RIGHT IN FRONT OF ODIN so that ODIN WILL SEE AND APPROVE OF HIM. His last words as he falls off into the void is that he wanted his father to be proud of him. I don't see him giving Odin shit so much as realizing by the second and third films that the only way for him to deal with Odin's increasing senility is to put him somewhere else where he can't harm anyone. And do you honestly think that Odin couldn't just get Heimdall to bring him back if he wanted to? Odin shows himself as being erratic and angry in the second film, and after the death of his wife, he loses his grip even more. He is willing to kill his ENTIRE PLANET'S POPULATION to avenge the death of his wife. Even Thor doesn't agree with this tactic, and has to go behind Odin's back to go with a different plan with a lower body count.
I do think that Loki is often portrayed in the comics as a downright nasty fellow. He is capable of great evil, and is often selfish, impulsive, and enjoys causing chaos. He is also written (at least in the MCU) as a complicated character, one who has been raised with a deep sense of duty and loyalty to his people, but who also knows that he is constantly in his brother's shadow, and there's a resentment there that comes out in conflicts between the two.
Both Thor and Loki have a very patronizing view of humanity and other races with less power. This mirrors a lot of how the British feel about, well, everyone else they colonized. Though Thor is nice to humans, his general consensus is "humans are weak and stupid but they are cute like a bunch of puppies and need me to protect them." Loki sees humans as weak and stupid and short-lived, but that they need overt control by a superior being to properly reach their potential. Loki didn't plan to commit genocide on humanity, he just wanted to rule the planet (haha, shows what he knows about humans).
These two sentiments are authoritarian in nature, but they are a product of their own society, which is also highly hierarchical and authoritarian with a king on top. Looking at how humanity segments things out and does stuff like voting and civic engagement must look like a weird backward custom to Asgardians.
Of course, I don't really think that the MCU thinks this stuff through all that much. I just think that, like most media, it's a reflection of our own society, which is often very authoritarian and patronizing towards "the masses" already.
These stories are written this way to hit a chord with the audiences that will watch them. Human beings can understand the story of two brothers fighting for the crown. Human beings recognize the narrative conflict of two people going down different paths to try and achieve the same goal.
Loki exists within the narrative to move the plot along in a specific way. He needs to act in the way he does in order to satisfy the story. Without Loki's interference, there is no plot. There is no movement. It is within the moments of chaos that he devises that the various notes of the story unfolds.
Does that make Loki a nice guy you'd want to bring to dinner with your family? Probably not. But does it make him a powerful narrative force in the overarching story? Yes, yes it does.