Think is, it's not only tax rates, it's what is and is not taxed. That varies from state to state. As example, my state does not tax groceries. The state 20 minutes from my house does. But they don't tax clothing items under $50 or $100 (can't remember off hand). Then someone has to make sure that the taxes collected are then paid to the states that they are owed. And when do you pay them? Monthly? Quarterly? It depends on the amount of sales often in those states. Or instance one of the companies I work with primarily sells within an 8 state region in the midwest. Do they make sales to California and New York? Of course, but not that much.
I'm sure that there will be services that will offer it, but how much will it cost? Credit cards charge about 2% just to collect money from one persons account and deposit it to yours. This is a much more complicated transactions, so how much is that going to cost? 1%, 2%, 3%? Both companies operate on margins of about 10 - 15%, meaning after the cost of everything is factored in that's their taxable profit. Add in another $10k a month to hire additional accountant or two plus additional legal services, even if a third party service collects and distributes there are going to be issues and someone has to balance the accounts and make sure the service is doing their job and in the case of one of my clients, you've just about halved their profits.