Some states do impose a sales tax on online purchases but in practice they never collect it because there’s no POS receipt for a transaction.
Some is collected, I pay mine in Virginia. When I first moved here many years ago I paid it and my lib neighbor told me nobody pays it. Later I had a debate about paying it with a Freeper (whose name I have forgotten) and lost. My argument was Constitutional but turned out to be incorrect. The next year I paid extra to make up for my nonpayments and I have paid it since.
IN practice, they never collect it because it is costly to figure out who isn’t paying, and people like to cheat on their taxes.
Many states have a line for it on their income tax forms, and signing the form without paying the tax is considered perjury. But again, it’s like shoplifting, if you are good at it, it is nearly impossible for a store to catch you, so they just raise prices for all the people who don’t shoplift.
Likewise, the state just sets their tax rate slightly higher to make up for the taxes that some people cheat on.