Because so much depends on the income tax based on the tax year. We calculate our taxes based on our annual income. Our qualifications for government benefits (subsidized housing, food, healthcare, financial aid for universities) are keyed to our annual income, as reported on our taxes. Many programs, even for richer people, also look to income: tax credits for electric cars, specialized retirement accounts, etc.

And because lying on your taxes is a serious crime, many private banks and landlords use those annual figures as proof of income for loans, credit cards, long term leases, etc.

It just pervades how we think of money, on an annual basis.