There are a lot of good reasons for STEM elitism. It's hard not to be resentful when you're spending all your time working your ass off to solve difficult math and physics problems, while the English or psych majors on your floor are out partying all the time and making too much noise in the middle of the night when you're trying to work.

It's also hard not to see those majors as jokes when we're forced to take some of their classes for distribution requirements. My English class had a heavy dose of postmodern literary criticism, which taught me that there's a whole culture of leeches in academia who just sit around with their thumbs up their asses trying to impress each other with nonsensical metaphors and big words they don't even understand. A physics major who knows something about quantum mechanics cannot help but want to spit all over the literary critics who abuse its terminology for avant garde metaphors that don't have one iota of actual fucking meaning.

There can also be glaring differences in the quality of the substance of these classes, and not just because of the difficulty of the field. In physics, you're almost guaranteed to learn and be tested about how the physical world actually works. Under any decent instructor, you can get an A in physics without knowing who Einstein or Newton were, as long as you understand the equations they developed. On the other hand, when I took a famous psychology class at a top university, I was appalled by the stupidity of the material. We learned very few genuinely interesting scientific results. We spent weeks learning about pseudoscientific bullshit like Freud, or other far-reaching theoretical speculations that never passed empirical muster. Tests were filled with questions about the terms from these defunct theories, the names of their originators, the years they were developed, etc. All meaningless trivia intended to help the dopes with easy majors get a good grade with a bit of rote memorization and then go out partying.

Of course, I know there are some intelligent, hard-working people doing interesting things outside the hard STEM fields, including in psychology and English. But on average, most majors are a bit of a joke by comparison.