Why do some words in Spanish that are masculine end in a?
Simply put, the rules regarding Spanish grammatical gender aren't absolute. There are always exceptions here and there.
In general though, there are two reasons you're seeing this: they're either loanwords from other languages that ignore the traditional rules, or they're established words that originally came from Greek a long time ago
*Note: Sometimes it's difficult to trace exactly but el papá "dad" is an abbreviation of padre and it's related to Latin [similar to el Papa "Pope"] - similar to baba meaning "father" in other languages. It's a fun little area of metalinguistics
Also el día "day" is masculine but it’s a very very old Proto word, not specifically Latin or Greek
As far as loanwords, it could be something like el sofá "sofa/couch" or el pijama "pajamas" which come from Arabic, or el karma "karma" which is a concept from Buddhism.
These are usually easier to spot because loanwords are some kind of approximation of an established word, something that either lacks a translation/transliteration or something that's directly from another language
The other big category here is words that originally came from Greek. Many of these words end in -ma, but they're all masculine, and many of them you probably know or will come to know:
el tema = matter, subject / theme
el clima = weather / clime, climate
el drama = drama / drama, theater [as a subject, not the place]
el fantasma = ghost, phantom
el enigma = enigma / riddle, puzzle
el estigma = stigma / stereotype
el dilema = dilemma, problem
This also includes pretty much any word ending in -grama. Any word ending in -grama is masculine and Greek-inspired:
el crucigrama = crossword (puzzle)
These are technically loanwords, but Greek is one of those languages that has had such a profound impact on Latin (and the Romance Languages by extension) that the "classical" words are kind of in their own category
Additionally - and this is a subset of words - but there are some words that begin with HA- or A-, that are technically feminine but take a masculine article.
They're more of a special case, done for the sake of preserving the sound of the words, so it's something of an imposed grammatical rule for clarity's sake, and so it's a separate topic
Here's more on those words if you want to read up on them!
kianamaiart
snowghoul
kaledo-art
germansingergirl
yousharknotpass
izzy806
theartofmadeline
dailybridgerton
