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?
Monthly income won’t tell the whole picture for people who are freelancers. Some professions have specific seasons where they make a ton of money, and other seasons where they make a lot less.