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?
That was my thought too. In Tech we do the same because of total compensation. Stock (options), bonuses etc. have a yearly cadence.
Also some places have a 13th salary, so yearly makes sense there too.