There are plenty of people switching machines that can be completely taken apart and replaced or upgraded component by component. Some other post around here mentioned eBay: plenty of completely modular HP and Dell laptops (and not even in bulk) for sale.
Most computer repair shops around here have disappeared because it's no longer the way people buy and use their computers. At least not enough people to keep those shops running.
Sometimes people upgrade because they feel like it. Doesn't always have to do with the numbers.
My opinion is that people switch machines more often precisely because they're not upgradable/repairable. I think if most people could go to their local computer repair shop to get RAM/storage/etc. replaced or upgraded, we'd have a lot less electronics waste, and consumer computers would last a lot longer.
At this point, software CPU and GPU needs aren't increasing all that much year by year, unless you're a gamer or do HPC. For the rest of everyone -- the majority -- a CPU and GPU made 5-10 years ago is still just fine for what they want to do. These old machines just need more RAM and sometimes more storage.
I get that it's hard to fit sockets for replaceable modules when everyone wants a super thin laptop, but overall this is a source of so much economic and physical waste.