1) You might not want to build the next Facebook. Many developers don't actually plan to start multi-billion-dollar companies. If you are an individual developer, or a small team, you cannot cater to everyone, you must pick the one platform most suitable for you. And then you can go create a niche app and make a decent living from it.
2) Even big and successful companies started on a single platform. A classic example would be Adobe: Photoshop was initially a Macintosh-only application, and has been ported only when it was already hugely successful. But actually Rovio is the best example that the choice of platform is extremely important: They have been hugely successful only when they started selling their iPhone app, and started porting to other platforms only when they were already extremely successful.
Hell, even Facebook, Twitter, Google started on a single platform: the web.