It's got nothing to do with cost, which is IMHO a complete canard. Spec for spec, MacBooks are actually quite competitive in the market in which they compete[0]. I'd
love to see a linux based laptop that is as well integrated and as well made as a Mac. It'd be great if it wasn't fugly too. If it's cheaper, that's a bonus, but in all honesty, I've yet to find a brand new laptop that costs less that £700 that isn't a piece of junk. Below that price, all sorts of noticeable compromises are made; from screen resolution to overall build quality.
[0]I do a lot of procurement in my current job and spec for spec, Macbook Pros actually sit in the middle of the market place based on cost and I argue that they are placed higher in terms of build quality than most other brands too. Extensibility is where they fall down massively, but then after 3 year, all our machines are retired anyway, by which time they are financially written off. For the record, I buy HP zBooks which are more expensive than Macs and in my experience not as well integrated with Windows or Linux.