People like you for example? People who are and have been so deep in the Apple box that you probably won't even consider a different browser because Safari is the best.

On the other hand: Chrome only became so popular because of their shady propagating of the software. Through freeware installers etc. Something you can prevent very easy on a Apple phone.

Also: the market is not so big. Even if they'd "take over" iPhone, it wouldn't make a big difference on the control they already have.