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):
Payment isn't always monthly, it is often every two weeks. So sometimes you get two paychecks in a month, sometimes you get three.
Compensation isn't just salary, even if you're salaried. Bonuses, stock grants, etc. might be done yearly/every 6 mo./every quarter.
Expenses aren't always monthly. If you own a place, you probably pay property tax which isn't due every month AFAIK. If you budget for vacations, holiday travel, etc., these are costs that vary wildly month to month, but have some stability on a yearly basis.