Because you have to pay sales taxes on the full value of the car, not on the fraction of the value that you got use out of.

Sales taxes are the #1 reason for why people who constantly lease do not just buy and resell cars.

This doesn't make a lot of sense to me, don't competitive pressures mean you implicitly pay for the sales tax you're not explicitly paying when you lease?

In my state (although not all) you get credit against sales tax for a trade-in. But it seems to me that's a primary reason why a dealer offers less on a trade-in than a private party would. Just because you're not paying sales tax doesn't mean you get the whole benefit.