Phrase origin: Why do we "call" functions?

Not a scientific theory, but an observation. New words propagate when they "click". They are often short, and for one reason or another enable people to form mental connections and remember what they mean. They spread rapidly between people like a virus. Sometimes they need to be explained, sometimes people get it from context, but afterward people tend to remember them and use them with others, further propagating the word.

A fairly recent example, "salty". It's short, and kinda feels like it describes what it means (salty -> tears -> upset).

It sounds like "call" is similar. It's short, so easy to say for an often used technical term, and there are a couple of ways it can "feel right": calling up, calling in, summoning, invoking (as a magic spell). People hear it, it fits, and the term spreads. I doubt there were be many competing terms, because terms like "jump" would have been in use to refer to existing concepts. Also keep in mind that telephones were hot, magical technology that would have become widespread around this same time period. The idea of being able to call up someone would be at the forefront of people's brains, so contemporary programmers would likely have easily formed a mental connection/analogy between calling people and calling subroutines.

Side-note: for me, at least, "salty" isn't anything to do with tears; in my idiolect when someone's "salty" it doesn't mean they're sad, it means they're angry or offended or something along those lines. The metaphor is more about how salt (in large quantities) tastes strong and sharp.

(Which maybe illustrates that a metaphor can succeed even when everyone doesn't agree about just what it's referring to, as you're suggesting "call" may have done.)

'Salty' in that context means 'bitter', ironically.

I would say “resentful”, “disgruntled”, “aggrieved”. “Bitter” feels like a more longer-lasting, somewhat less emotional condition to me.

And I agree that it has nothing to do with tears. The actual etymology stems from sailors: https://www.planoly.com/glossary/salty

True enough, there are endless subtleties to language (English in particular) that make words simultaneously vague and extremely specific.

Wiktionary defines bitter as "cynical and resentful", which doesn't quite capture the "more longer-lasting, somewhat less emotional condition" part of it.