Even trying, doing it securely would be hard.
In the sense that precision takes time and money, but mostly the former.
Starting on a Linux base gets them a lot of time saved, but as soon as they start having to write custom code (and they'll have to write a lot of it to re-adapt Linux to mobile systems) they're back in the same boat.
Essentially, mythical man month. There's no amount of programmers you can throw on a project to produce a secure, complex OS in a week's time.
Extrapolate to Android still making major security rearchitectures after a decade? Good luck starting back on Step 1 of that journey.