Patents are probably part of it. Another motivation may be that Google wants to avoid the Android ecosystem to spiral out of control. Android phones vary widely

Patents are probably part of it. Another motivation may be that Google wants to avoid the Android ecosystem to spiral out of control. Android phones vary widely in quality, both with respect to hardware and software (thanks to vendor 'extensions'). Maybe they are hoping to produce an Android phone under tight control, to set a benchmark for the rest. Also, Apple has shown that piles of cash can be made on good, though slightly expensive margins.