Because it's easier to compare yearly salaries. If you say “I’m making 50k” then I can assume it's yearly (if it's monthly then good for you, but that's overwhelmingly not the case). Otherwise you need to specify the pay frequency and do math.

Same with net/gross. I can assume your yearly salary is given in gross because net introduces a bunch of variables with tax rates based on location/deductions that makes it a fairly meaningless value to compare against. Not to mention your net monthly income may change in a year as you hit certain tax limits like the max social security tax limit.

Some people may also not have consistent monthly earnings so averaging income over a year gives a more accurate picture of average income.