Because they don't have a choice, so they don't expect anything else. Only the technical folks know how and where to buy a clean computer. Hell, the first computer the younger generation bought themselves was most likely already loaded with a ton of such crap, and that sets the baseline of their expectations.
Then set up a company that make fresh PC with windows installs without crapware and you have a business model right there. Choice is just a matter of people starting doing it. Or you know, buy a Linux computer (sure, the choice is not great there either, but it's something that a few manufacturers offer online: Lenovo, System75 and a couple others).
People like PC World in the UK, as its the closest they have top a normal shopping experience.
I always told friends to go to the small independent shop over the road where they will get a better deal. But then they have to speak to someone who will baffle them with jargon. Its far easier to see the 40% discount offer (from the initially high prices) as some sort of bargain. I have wasted hours uninstalling crap from PC World machines. One came with 3 firewalls and it wouldn't connect to the internet.
