Because they are different taxes managed by different levels of government and none said governments have to make sense.

You are supposed to declare untaxed purchases on your state tax filing and pay a "use tax". I don't think many people do.

Tariffs aren't a sales tax. They are an import tax (designed to persuade you to buy products internally produced, or pressure the international seller to come to a trade agreement).

International shippers don't charge you your State Sales tax because they aren't involved in your system and don't typically have a way to remit the sales tax portion back to your state.