I would say (which means: IMHO) that it's because the UI "is" the OS to 99%+ of the world. We developers, geeks, tinkerers, etc. know what an OS is. But to the end user, it's what they see on the screen. It's what they interact with. That is the abstraction we attempt for the vast majority of users. There is no need for most people know know how something is implemented. They just use it. The same applies for just about every other thing in existence. We use things. We consume things. We don't need to know how they are built to use them. Sure, some of us care, but most don't. Just talk to non-nerdy types and find out how ignorant (and I don't mean that in a pejorative way) they are when it comes to technology. Just as in Android and iOS. Those users don't care about a Linux kernel with a Dalvik VM vs. a BSD microkernel running applications compiled mainly in the Objective-C language. They care about cost, public image, features etc..

I'm just throwing this out there for thought. But, I don't hear a whole lot of us talking about who's microwave oven's firmware is implemented better. It just doesn't matter to the user, as long as it works. It is usually the style and ease of use from an interface standpoint. Same for cars to most people. They don't care about how the technology under the hood works. If they get the gas mileage, style, price, automatic or manual etc. they want, then that is what they buy.

So, to most people a Car is the Make and model. A computer is the UI and perhaps the software available for it. That's all.