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 Québec, the default frequency for being paid is every two weeks. Big salaries are mostly stated as yearly salary, while more menial labour is often by the hour.
I don't have an explanation, it's just what it is. But I heard over that being paid every two weeks was because people were so bad with money that they would starve every month, so they started paying us every 2 weeks instead. Go figure.