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?
Can't speak for the US, but here in Germany there often aren't 12 monthly salaries to a year. Many people get a Christmas bonus and/or a summer bonus, but just as many don't. Personally, I get paid about 13 1/4 monthly salaries a year, so telling you my yearly salary would be more accurate than the monthly amount.
Same in Italy