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?
Personally, I don’t get paid every month, I get paid every two weeks, which means that some months I get paid twice and some I get paid thrice. Stating an annual value corrects for weird shit like this, and it’s going to be consistent since it’s probably how it is being tracked in the employer’s accounting.