An OS operates hardware, hardware needs drivers, drivers need testing, this is expensive and time consuming and not very possible for all hardware, even if the parts claim pin compatibility.
Hardware manufacturers compete with each other for market. If every handset is the same compatible hardware, that just leaves build quality and price as the variables and they are orthogonal.
That means hardware people also like to add features like GPS, compass, different cameras, expansion ports, heartrate monitor, fingerprint recognition, dual-sim, etc. etc. and we're back to drivers and testing.
The PC market used to be the same way -- you had Commodore, Coleco Adam, Apple, Ti, etc. But eventually the market settled on a standard (x86 architecture), with multiple OS environments and applications available on hardware from multiple vendors. Of course you still have outliers. But I'd think that at some point a new player would come into the market, make a bunch of handset models with off the shelf features, and ship the environments separately. Just like IBM did with the PC, and the clones followed.
Maybe but there is much blood on the floor of those markets. They rallied around one OS, if you recall, and that OS manufacturer was successfully prosecuted for exploiting their de-facto monopoly. Consumers were definitely harmed.
Dell, the leading hardware supplier lead the race to the bottom and now flounders. PC commodity markets are cut-throat and it is Intel that makes the real $.
Hardware manufacturers compete with each other for market. If every handset is the same compatible hardware, that just leaves build quality and price as the variables and they are orthogonal.
That means hardware people also like to add features like GPS, compass, different cameras, expansion ports, heartrate monitor, fingerprint recognition, dual-sim, etc. etc. and we're back to drivers and testing.