>I don't understand why mobile systems do not attract OS builders.

My guess would be that it's a continuously moving target. There's no point in spending years working to support some weird integrated wifi adapter+battery controller when by the time you're done the hardware is already obsolete and no longer being manufactured. Repeat that for every device on the phone. The only ones who can keep up with that pace are the manufacturers themselves. It'd be different if there was some kind of standardization that would make the effort worthwhile, though.