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?
I mean, you just basically answered your own question. People get paid hourly, weekly, every 2 weeks, monthly, and some even per sale (ie. Realtors) so the only way to have a constant measurement is yearly.
Why not monthly? It seems the smallest unit to encompass them all, and is fairly standard.
Monthly makes sense also since most bills are monthly.