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?
In Czechia, it's definitely normal to say salary per month.
Same in Poland, it's definitely more common to discuss monthly salaries. I had to adjust when moved to Germany, as almost everyone uses yearly base. Though I would say it makes sense considering yearly bonuses etc.