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?
Where do they do it otherwise? In Australia it's also yearly.
People might also get bonus so in some sense you get paid once a year.
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.
South Africa do it monthly