Some huge percentage of that is because many if not most laptops shipping today have wired everything down. You can't even update the Wifi chip in most laptops today, even though the whole reason we put Wifi on a daughtercard in the first place is because we knew the standards were going to have to move...
Meanwhile, I'm typing this from a five year old laptop that I would have happily upgraded by adding more RAM if possible, and I know a lot of people that would be more than happy to do the same, since CPU technology hasn't really been improving in leaps and bounds and neither has laptop design in the past five years.
Let's not be afraid to call it what it is: It's planned obsolescence. They want you to have to buy a new one in 18 months when they ship version 2.