Mostly because the app still brings in money. Anybody with a basic understanding of mobile coding could easily duplicate the app in 30 minutes thus streamlining a bunch of competitors I would rather not have.
Obvious throwaway account but 4 years ago went to odesk.com as I had a super simple idea to make an app. I believe spent less than 2 hours paying a developer $50 an hour making the app. Had it up in less than a day.
Couple years later the app has made close to a million dollars with me pocketing about 60% of that and the other 40% to the ad company.
It's one of my favorite projects because it was so simple and literally took less than 2 hours but I was able to pretty much make a 1000x on it which to this day is better than anything I have ever done.