Because they were rubbish? HP, Toshiba, Asus, whatever, by year 3 they were all dying, and I'd have to nurse them along for as long as possible until I could shell out for another. Broken hinges, cracked cases, replaced HDDs, many Windows re-installs, etc. Until 2012, when I got a MacBook Air, which I used daily until this year.
My desktops survived far longer by virtue of continual upgrades. By the time I replaced my previous PC, the case was the only original part. My current PC is escaping that by virtue of my not using it. It's not even plugged in, it's under a pile of boxes. Eventually I'll want to play Train Sim World 4, and dust it off, but until then, I've got no reason to bother finding it.