In Florida - and I suspect in most other if not all states - the sales tax is technically levied on the buyer, not the seller. The state just forcibly enlists a retailer to act as its agent in collecting and remitting the tax as a condition of having a business license and the permission to conduct commerce in the state. So in a normal retail transaction, the seller isn't tacking on the sales tax to your bill as a means of cost shifting something else from them to you. They are acting as an agent of the state in collecting the sales tax that you, the buyer, are legally required to pay.
That is why in Florida - and again I suspect in most of not all states - when a purchaser buys a product, any product from anywhere, and sales tax is not collected, that buyer is legally obligated to file paper work and remit the tax to the state.
Of course consumers get away without doing this all the time. Other than conducting invasive fishing expeditions in to every private citizen's life and financial transactions to find suspicious purchases, there is no good way to do it.
Business entities? That's another story all together. The state knows exactly who you are and what business you are engaged in. And they know which businesses are particularly infamous for buying high-dollar value equipment out of state from tax-free outlets like Amazon, and not reporting the sales. Photographers are a favorite target.
I know at least 3 that have routinely been audited out of the blue. On the first go around, they get a nice little letter from the state reminding them of the requirement, and inviting them to submit a detail list of all equipment on hand, where it was bought, the purchase price, tax paid at point of sale, or tax remitted to the state after the purchase. There is also a "get of jail free" voucher with a one-time grace period to submit any back tax due without penalty or interest.
Second and third audits, are noticeable less friendly.
I can fully understand why an entity that does not have a physical presence in a state would not want to comply with state sales tax filing requirements. I don't know about other states but in Florida, there is a state-wide base rate of 6%, and then each county can add additional taxes on top of that in what is called a local option sales tax for specific local needs and projects. It's usually in the range of .5 to 1% And sales taxes are all assessed and collected at the county, not state level of jurisdiction. So just in Florida alone, an out of state entity would actually have to maintain as mainy as 67 different tax tables that would have to be updated every time there is a county election that changes the local option rate, and would have to file quarterly returns with 67 different county tax offices.
Multiply that by the number of taxing authorities all of the U.S. that can set and collect local sales taxes, and the scope of the problem increases at an exponential rate.