You're thinking about this the wrong way around.
Suppose the user has $800 to buy a new laptop. That's enough to get one with a faster processor than they have right now or more memory, but not both. If they buy one and it's not upgradable, that's not worth it. Wait another year, save up another $200, then buy the one that has both.
Whereas if it can be upgraded, you buy the new one with the faster CPU right away and upgrade the memory in a year. Manufacturer gets your money now instead of later, meanwhile the manufacturer who didn't offer this not only doesn't sell to you in a year, they just lost your business to the competition.