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?
As others have mentioned, a few possibilities (I’m in the US, not sure how specific this is):
The tax point is probably the biggest one. People just want to know what tax bracket you fall into. And it corrects for seasonal variations.