Well, one might think that one reason those other companies do that is because it's cheaper for them. But it's not entirely clear why. The only tool required to apply a bolt is a driver, which requires no training to operate. Also, you might think that there are reasons to use the rivet in those other kinds of knives and that those reasons aren't at work in the case of Spyderco (maybe rivets work better for pinning several tools together, as Victorinox does, or maybe the Case and Opinel people don't want to adulterate they're very old designs, etc.)
I'm aware that making knives this way "keeps people who should be allowed to own tools from taking apart their knives". This doesn't strike me as a design virtue.