Is it because they're all running the same OS and making better hardware doesn't offer enough differentiation?
They run Linux and FreeBSD kernels really well with all hardware components working out of the box as well. Compared to my last experiences with Dell and HP, I'm sticking with Lenovo and Apple for good.
Some even try to compete on product design (HP Envy?), but they fail to match up to the attention to detail that Apple is known for. My guess is that they have outsourced most of the process to their suppliers, which means that they have less of an influence on the design details. Apple spends a huge amount to keep an in-house design team that not only does the superficial design, but also the layout of components, figuring out which materials to use and how to use them, etc. From the film Objectified: http://vimeo.com/7827217
For instance, all PC laptops have a big external power supply with long cables with some kind of velcro strip on order to tidy the cables It's been like this for 20 years. Apple probably uses the same internal components in their power supplies, but they've designed one with a nice magnetic connector, a detachable power cable and nice "hooks" for winding the cable. They've had this design for at least 10 years. This is probably not a deciding factor when people buy a laptop, but it's a nice design detail that Apple decided to spend the money on developing, whereas other laptop manufacturers never thought about it. Or, more likely, a lot of designers have thought about it and has have had their ideas axed by the business CEOs.
A less elegant solution is the breakaway cable used on the original XBox controllers, which would detach while leaving the "main" connector still in the laptop.
Also: What stops PC manufacturers from creating their own OS? (Obviously not from the ground up but they could create a Linux distro ála Ubuntu.)