I'm not denying you can get equivalent, even better, knowledge and experience otherwise. One of the best devs I know has a degree in philosophy. However, some of us know what we go to school for and we make it worthwile.
Sure, it's no 100% guarantee, but it should at least mean you're familiar with the domain enough to find the answers you need.
You said yourself it entails loss of sanity and time - agreed, completing one shows discipline, character, time management skills and enough sanity that you can afford to lose some. Again, it doesn't mean people without a degree don't have these things (self-education can require some of them even more), but that's one way to demonstrate them.
Finally, there's different degrees, mines are from MIMUW so they do prove quite a lot.