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?
Except plenty of people don’t get paid monthly? I’ve been paid weekly, every other week, twice a month. I’ve been paid daily, I’ve also been paid per project. Once I was paid whenever i asked. Focusing on yearly salaries offers a convenient reference point.