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?
maybe tax related since taxes are based on annual income. if you are not hourly/salaried and you are self employed/freelance/contract your income will vary from month to month. annually seems like it can be more accurate across all those groups