Many important parts of checkout are open-sourced via our jQuery.payment plugin [1]. It allows you to build your own checkout flow from component parts. As for open sourcing (capital C) Checkout entirely, we're working hard to provide a high-touch, consistent experience for checking out and we haven't been able to reconcile that with a bunch of small (maybe less thought-out) variations on the same thing floating around. We're always trying to think about ways to make this stuff better, though. We're definitely not against the idea, we just wanna do it right (i.e. allow non-programmers to customize without making things work incorrectly or look bad).
1. https://github.com/stripe/jquery.payment