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):
The tax point is probably the biggest one. People just want to know what tax bracket you fall into. And it corrects for seasonal variations.
Mortgages neither. Mine is accelerated bi-weekly, meaning I pay essentially 13 months a year... It shaves a wooping 3 and a half years on my 25 years!