It’s because what they’re doing can sometimes provide temporary relief and when it works, it works fast. An underlying cause has made some inflammation, they stretch things out, relieve some pressure in places that shouldn’t have pressure. But they’re not fixing anything, just letting your body get back up to barely functioning until the underlying cause rears it’s head again. Messed up discs are their bread and butter, but they’re just resetting the house of cards you call a back.
Actually fixing the problem is a big, expensive, scary, painful deal and (US) chiros let insurance companies off the hook for a long time.