Designer of Forge here.

We're aiming to solve the problems of the non-techie market. People who just want to sell something online shouldn't be worrying about payment gateways, payment processors or designing a checkout process. Forge handles that.

With Stripe, it's fairly easy for a programmer to write their own payment processing backend and build a checkout process for it. We're not trying to serve the people that have the development skills to use Stripe. We're going after the people who use PayPal Buy Now buttons.