Krita is almost 20 years old and is more focused on painting than on image editing – but personally, I use it for both, liking the UI much more than that of GIMP.
Because it's possible and someone wants to. Same reason why we have multiple Linux distros, multiple databases, multiple browsers, multiple text editors.
Are you surprised that open source software in general is duplicated? Or is this specific just to image editing software. If yes, what makes image editing software special so that having a second option is surprising?
Yes, open source promotes collaboration. It also promotes forking and starting new projects.
Obviously they both manipulate images so there's lots of overlap in features, but the idea of painting or drawing in GIMP seems really alien to me. I'm sure the interface and pen support was even worse when the Krita project was started.
There are actually more than two.
I'm actually more confused by the converse: why do people keep using and recommending Gimp when Krita has existed for decades and is so much easier to use?
Krita is actually quite old. The reason you haven't heard of it is probably that it's more focused on digital painting than on general image manipulation.
The fact that you think there's only one open source image editor out there is fascinating.
Krita on the other hand is designed for drawing and painting.