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?
I think it's probably one of those things that is stupid until you reach a point of financial success or fall into groups that consider your financial wealth important. Why it's a thing is probably because we pay our taxes once a year and that's when it's laid bare and you see how much you made. So after 10 or 20 years you kinda know what 50k a year is and if someone is talking about making that much you can understand the lack of money they have. If you friend tells you that, don't ask them out to expensive things unless you're going to pay the bill.