I think this is mostly a US thing. Why use yearly salary? You’re not paid once a year, are you? Most likely once a month. Referencing monthly salary makes much more sense.
“I’m making 50k”. Great, now I have to guess - dollars? Monthly? Yearly? If yearly then what’s the monthly paycheck? Net? Gross?
It’s not just a US thing. I’ve never actually thought of this until this post, but I’d think it’s because taxes are done annually.
Your employer says they’ll give you X amount a year, but you receive X-Y into your account. It’s easier to talk about X, then to worry about how Y fluctuates.
It also makes it feel as if you’re making more money. Raises for a year sounds better than when you divide by 12 and get the monthly.