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.
Until you have people who get a yearly bonus. Or 13 or 14 monthly salaries a year, which is quite common in Germany (basically a bonus, but the employee is entitled to it).