Yes, those platforms may handle sales tax calculations automatically for you for their specific platform. That's only the very first step. Merchants will then need to report and remit their sales tax to each state. If those platforms plan to handle the entire process, trust me they have a ton of work ahead of them if they decide to handle it in-house.

Arguments like this annoy me. Someone has it figured out so it's OK for these complex tax implementations to exist. No, it's not OK, and it's not a solved problem. We recently implemented sales tax collection using the TaxJar API combined with Stripe. It took at least a month to integrate. These integrations are huge drains on business productivity. Government has to stop creating these complex rules. Create simple rules and stop treating everyone's time with such reckless abandon.

We don't. We're one of those mom-and-pop businesses that uses a self-hosted shopping cart, and use Braintree as our payment processor which has no sales tax recording system that I'm aware of.