Personally, I've never gone ahead and made my own Japanese learning app or anything like that, but while studying I do pretty regularly think, "man, this would be so much better if it did X; I wish I had an app to do Y", etc., etc.
Because of Western attitudes towards Japanese pop culture, a lot of people learning Japanese are...well, they're gonna be kinda nerdy, frankly. And that means they're more likely than average to have the sort of skills that allow them to build (software) tools to help them study.
Throw in that learning Japanese, at least for Westerners, is a monumental task mostly driven by intellectual curiosity (rather than for, say, economic opportunities), it'll attract the sort of ambitious and intellectually curious people that'll actually endeavour to make their own tools, with the necessary skills to do it.