I have a little gem at home called a "Power Computer PowerTower Pro", this was a 266MHz PC made by a company OTHER than Apple, and ran Mac OS 8. This was Apples short, failed experiment on licensing their OS to other manufacturers. The problem was, just like the problem with Android in some cases, is that the system was not made up to the standards of the OS maker, and Apple couldn't support it! They got support calls that turned out to be hardware issues, and the manufacturers wouldn't help their customers. Furthermore, they found that some of these models were built so sub-par spec wise, that there were complaints about certain features of the OS not working at all! I upgraded mine to OS 9 but only after upgrading the RAM. I like my PowerTower, but it was nothing near the reliability and performance of the Apple Macs. In fact, if you bumped into it, it crashed, just like that. Literally, completely locked up. It also had this funny thing (and common problem) that it would not boot unless there was some sort of disc in the CD drive. It was also considerably bigger and heavier than any Mac, and it was beige in a time when Apple was trying to prove that Beige was dead!
All in all, Apple didn't like having their product on a PC like that, it wasn't their business model. Macintosh was supposed to run on streamlined high performance machines (back then, the specs of those machines toppled most PC's in a BIG way, the original PowerBook was the fastest laptop in the world at the time, equivalent of today's macbook having a quad-core i7 and Mobility Radeon HD5870 or something), if companies wanted to build stuff like this, they could use Intel processors and run Windows.
Another big issue was driver support, mine had a Twin Turbo brand graphics card, it had one version of the driver... ever... Apple didn't make it, and that small group of non-apple hardware manufacturers didn't warrant a big need for hardware makers to make Mac OS drivers. Apple used ATi cards in their machines.
All in all, Apple only did it for I think 2 years, and were very unhappy with the outcome. Now, most Android devices have some sort of modification to make it cater to THAT device, which is possible with an Open Source OS (only the Nexus phones are Vanilla Android), with iOS it wouldn't be. Not to mention, what is the biggest complaint about Android? That it's not streamlined enough, it's choppy, it's not as "spit and polish" as iOS. That's not COMPLETELY an Android issue, it's an issue of tons of hardware manufacturers trying to run the same OS! Look at the "spit and polish" difference between Windows and Mac! When you make an OS cater to ONE device, you can make it scream, that's what Apple has ALWAYS done, and that is what they will continue to do.
PowerTower Pro circa 1998 - NO USB, NO included modem, Slow
Came out the same time as
iMac, circa 1998 - Fast, ONLY USB, Built-In Modem for the "Internet Mac"
See why Apple was unhappy with third party manufacturers?