Payment processing services could certainly build out their own solution to collect and remit sales tax, but a lot of merchants sell on a variety of platforms and channels. For instance, a merchant might sell on Amazon and their own eCommerce site powered by Shopify. They might also have an ERP system as a "single source of truth" to record all of their transactions. Each of these platforms collect sales tax differently, possibly providing automatic calculations or simply leaving it to you to provide your own rates. This can get complicated very quickly and a payment processing service only has insight into the data you're passing over to them.
That's why companies like TaxJar and Avalara are focused on sales tax -- to integrate with every shopping cart, payment processor, marketplace, accounting, order management / ERP platform out there to combine all of your transaction data and determine how much sales tax you need to remit in every state.
That's why companies like TaxJar and Avalara are focused on sales tax -- to integrate with every shopping cart, payment processor, marketplace, accounting, order management / ERP platform out there to combine all of your transaction data and determine how much sales tax you need to remit in every state.