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?
This must be very regional. Additionally, I’d bet a lot of this might depend on industry.
Someone who’s hourly might have fluctuations in their hours over a set period of time, like a month, or even week to week.
Seems like a number should always be coupled with a unit.