What do you think volunteers do here that is "out of selfishness"? Just going by one example, me, let's see what my contributions have been (not counting the stuff I get XP for).
You see that "Reputation: -12 (+0/-12)" that shows up after you've voted, or on your own nodes? I did that - it was something I thought would improve the site, I got in there, made a patch, got it tested out for a while, it was rejected, improvements suggested, and finally repatched and applied. I felt that it would be an improvement, and drove it through completion. If I had waited for others to do it, it might not be done yet.
Both of those examples had to run through the gods here, so may not be directly comparable, though if your wanted app required more ways to display nodes, it would definitely be more work, and you'd need the gods' approval on any code changes that you would make. Note the highlight. If it's your itch, like the above changes were my itch, you do something about it.
I've also contributed to perlmonks off-site: Chatterbox Addicts not-so-anonymous/Tanktalus' CB Stats relocated (temporary, I hope), which is coded completely independently of the gods (their only input is that I'm following site policy, which I think I've bent over backwards to accommodate, thus everyone, or at least most everyone, is happy), and last hour of cb is back, which required tye's intervention only to modify the owner of last hour of cb so I could update it, but the code that does all the real work is off-site, thus the gods don't concern themselves with my implementation (only my output).
The key here is not that I did these things. It is that I wanted them done, therefore I did them.
If you want a smartphone app, go ahead and write one. I remain unconvinced that there would be any money in it, but, then again, I'm too cheap to get a smartphone (they're cool toys, but I can't justify the expense). And, though I'm unconvinced, I'm not going to try to stop you or discourage you from doing it. Let us know when it's finished!