That every developer simultaneously develops for iPhone, Android, Windows Phone, WebOS, Blackberry, Meego?
Developers aren't so cavalier as you about suggesting that you port your code to multiple platforms because we do know how much time, effort and platform-specific knowledge it takes to build a decent app.
Many iPhone, Android, Windows Phone apps are written by single developers that can't take 6 months per platform to learn how to code for it.
For most of us making a bet on single platform is the only viable strategy.
Bad strategy? Maybe. The only one possible? Unfortunately.
You would be pissed off too if your business failed not because the owner of the platform you bet on built a better product than you (and with their money, teams of programmers, marketing and ability to advertise to users of that platform they do have a huge advantage) but because they just used their monopolistic control over their platform to set business terms that are unheard of on other similar platforms and kill your business without them needing to lift a competitive finger.
I'm sure you'll tell me that Apple has the right to screw everyone on their platform and if I don't like it, I should just use another platform.
And that's exactly what I'm hoping people will do once they realize that Apple not only has the power to screw their developers and their users but is actually screwing them, as this on-going saga of 30% monopoly tax on content sales shows.