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 can't even compare wages with my partner if we have to go by monthly rate.
I get paid per month, but in May I get an 8% bonus, so my monthly payment is not the same throughout the year. Then my partner gets paid every 4 weeks, and receives bonuses based on company performance in those 4 weeks. So every payment is different.
Per annum is the only way we can compare our salaries. And that's in the same country. Now try international, and it'll really difficult otherwise soon.
I live in the Netherlands.