That’s a devilish engineering question. If you are truly framework agnostic you limit how much work you can do for your users, because a lot of the work happens inside of the framework. We decided we’d commit to doing the work so our customers didn’t have to.

E.g. most web apps have some endpoint that the client calls to initiate a checkout session by calling their payment processor’s server SDK.

How many times has the following code been implemented because no payment processor ships a route handler and a React hook (pardon my React-brainedness)?

Someone has to do the work to get that checkout request from your frontend to your payment provider and then back so you can redirect. In a just world, that your processor’s SDK would handle that work. Otherwise it falls on your plate.