To be fair, if any company hired you based just on your app, it would be unfair to you and the company. Most software companies thrive because of generalist SWEs who need to demonstrate consistent performance.
It might be that the app was a foot in the door but might not be applicable past that initial point
I mean, you maybe wouldn't just send an offer letter to someone who wrote an app, sure. (But depending on what they wrote, maybe you would. Something reasonably large and open-source, where they ended up incorporating a substantial amount of code from other contributors?)
But if someone's got 30kloc sitting there, why wouldn't you look at that body of work instead of starting from scratch with "please write FizzBuzz on this whiteboard"?
Because reading someone else's proprietary code exposes the company potentially to liability? Not just claims of IP theft, but there would have to be legal vetting that you have the right to show that code in the first place. Also, I have no way to confirm how much of the code you wrote yourself.
It might be that the app was a foot in the door but might not be applicable past that initial point