Because we have to tax something. Taxes are for ongoing services. You have to keep paying them because roads keep getting potholes and kids keep being born and needing education.

Only taxing income, or sales, or payroll, or imports, or property, or capital gains, or whatever would disproportionately affect one population or another. Spreading out to multiple sources of taxation is an attempt (with mixed success) to make sure that everyone pitches in. Property taxes in particular (assuming a reasonable homestead deduction) are a good way to make sure that rich people pitch in.