Either A) because none of the existing apps are good enough, or B) note taking is pervasive and diversive enough that there is no one app to satisfy them all.
I think its a mix of both with a lean towards A personally. Knowledge workers are increasing YoY and nearly all of them take notes. The market is huge, and its quite normal for new(ish) markets to see a large number of competitors before the winners take over and the other 90% die off. To your other question -- Google Keep / Apple Notes are good for rudimentary use cases or small numbers of notes, but otherwise don't really compare to the popular note taking apps that have popped up lately.
I think its a mix of both with a lean towards A personally. Knowledge workers are increasing YoY and nearly all of them take notes. The market is huge, and its quite normal for new(ish) markets to see a large number of competitors before the winners take over and the other 90% die off. To your other question -- Google Keep / Apple Notes are good for rudimentary use cases or small numbers of notes, but otherwise don't really compare to the popular note taking apps that have popped up lately.