It's not cost effective for an OEM to ship Linux. It limits their hardware selection for a start.
Assuming an OEM had to decide to ship Linux on all their motherboards and be compatible with all their products, that might be the case.
But that isn't the case..
It's all about sales and marketing.
If Dell could sell a consumer laptop with Linux on it, but Linux wasn't compatible with their corporate laptop chipset, AND that consumer laptop with Linux sold really well, Dell wouldn't care. They'd sell it if it sold well.
If it wasn't compatible with all their laser printers, but the laptop still sold well, they'd still sell it. After all, it's about selling product.
The issue with Linux on OEM machines is simply that they haven't sold well enough.
I think a lot of that is because it has been kind of advertised as a Windows replacement, but it's not. It's its own OS with its own benefits and drawbacks.
At work, people were psyched when we started to provide iPads for calendar/e-mail and notes. And then people started getting them and some of them were disappointed. The iPads didn't run all of their macros in their Excel docs. They didn't connect (or connect well) to the iSeries...
Fact is, the iPad is great for what it is, but those people wanted a Windows compatible device too. That project was still a success tho, because the people who were happy were the people who mostly needed e-mail/calendar and notes (i.e. managers, administrators and their assistants). ;-)
If you want/need Windows, you won't be happy with Linux.
If you want/need a device that can do what Linux can do, you can be happy with it.
Good news is that Linux does a lot that people need and works on a lot of hardware really well. (Typing this on a DELL XPS M1210)
But it doesn't do everything, and some people just want Windows.
That's OK..
As the market goes more web based, it gets more and more open for Linux (and OSX). And choice is good for everyone.
Heck, Windows Server 2012 has several new features that I think were inspired by the Linux server competition. I think that's great. 2012 is much better as a result of Linux. (IMHO) ;-)
desiv