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.
If you can assume the framework someone is using, you can deliver a powerful, comprehensive integration.
If you instead try to build a generic one that anyone can use, then it's both hard for you to build it, and hard for everyone to integrate with it because it's generalized.