Because note-taking experience is subjective, and there are no solution which fits everyones taste and purpose.
I'm not so good at ballparking operational expenses, but Supernotes 2 probably needs somewhere around 2000 users to break even, which probably isn't hard to achieve.
After that, the founders are fully supported to pursue their passion. Sounds like a great business model for someone who wants to do this, and I think this is why you see a profusion of utility apps with ~$10/mo pricing models on the market today.