Because nuanced discussion often requires context where colloquialisms typically don’t. You could absolutely say “jobs that require no specialized training at the outset”, but if you’re writing a paper or having a technical discussion in a labor field, that is really cumbersome. It’s easier to pick a context-appropriate one or two word solution. This is generally called a term of art.

It’s worth looking up “term of art” for a few more examples if my description didnt do it for you.