Bitcoin has been developed for over a decade now by a large group of people.

But if you read the original posts and software releases from around 2009, you can see that it was pretty much a single, very talented, developer. This is not too uncommon in the world of open source.

I mean, if you have a look at the original release of the software, it was just three files, and not a huge amount of code:

Yes, the idea was brilliant, but it was also based on a lot of previous works, as usual in science and technology.

It's perfectly possible for Satoshi to be one person.