Needing a way to track finances is a problem older than writing. It's an almost universal problem, and everyone has their own ideas on how it should be done.

It's also a complicated idea, and when you look at existing solutions, they almost always have a huge number of features that any given individual won't need, which makes it harder to figure out what you're doing.

If you have a background in programming, and none in finance, it might be easier for you to start from what you understand, and build up an app that does what you need first, then add features later as you understand the problem space more.

No idea if it's still the same now, but banking account management, both from the consumer and the bank side, used to be very common example use cases when teaching OOP. Maybe some of those example projects grew into hobbies which grew into OSS projects.

Along with note taking and todo apps, it's low hanging fruit. It's a problem most people / developers can understand without having to do a lot of studying, and it fits in people's heads.

But it's also often an example of people not willing to learn a new tool, or not willing to pay for one, so why not build a new one? How hard can it be?