I mean I paid for it like I would anything else I wanted. They charge a tax at checkout. So if I buy a house and pay the whole thing off, why do I still have to pay taxes on said house when I paid the whole agreed on price in full? It would be like me buying a six pack of beer I pay for it and tax at checkout. But then timely I have to keep paying taxes on the beer even though paid in full?
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.