MIT license is a very simple license that basically has no restrictions which makes it perfect for a decentralized project such as Bitcoin that doesn't want any obstacles in front of its development, improvement and adoption.
This way you can create proprietary software on top of MIT licensed Bitcoin without restrictions but if it were something else like GPL they would have to do everything from scratch.
I agree with you that MIT license is very permissive for decentralized project like Bitcoin, but I'm very curious to know that why Bitcoin core wasn't launched in public domain? Let's suppose if the core was launched in public domain then wouldn't that be more decentralized because in that case no-one, not even the developers could have any control over it, and anyone could easily modify and add new features to it without any permission.
I'm asking this just out of curiosity and truly I don't have much knowledge related to that stuff. I know something about the licenses like they provide permissions to anyone who wants to contribute to the code, or the developers who want to add some good features into the code, and in that case isn't public domain more permissive?
I think the Creative Commons Zero license would be even more permissive and easy to use than the MIT one, and that license doesn't require the attribution to the original creator. If the reason was permissiveness then both public domain, and CC0 license would provide more freedom.