The answer is because the Sales tax is formally the Sales and Use tax. The tax is generally on the use of a purchase, which is facilitated by the services of t

The answer is because the Sales tax is formally the Sales and Use tax. The tax is generally on the use of a purchase, which is facilitated by the services of the State beyond just the sale. It's collected at point of sale as that's easiest. If not collected at point of sale, it's still due as a Use tax.