It's because you treat it as if it's all or nothing. You can effectively allow apps to cooperatively schedule themselves, but also make sure `while(true){}` doesn't infinitely loop the system. You can e.g. timeout each app, or if you feel very experimental, avant-garde etc, you could try detecting loops, and put a very generous timeout (since you'll miss some loops like `i=0;while(true){i+=1}` depending on the algo you use to detect loops). There are tons of other schemes, algorithms. Most of these ideas are implemented somewhere, but 99.9% of computer users using linux/OSX/Windows don't experience them.

For the programmer, it's most convenient when the isolation between unrelated programs is maximal but isolation between related programs is minimal; but for the user it's the most convenient when programs are maximally isolated in terms of computation, but not for other things such as storage/permissions etc. Practically, this requires a complex scheduling mechanism that is more preemptive than cooperative, but also cooperative a little. E.g. in linux, it's not particularly hard for a rogue program to bring the system to halt with something like a fork bomb (especially if swap is turned on). Even if you have a preemptive scheduler, if your program has 99% of the threads running in the system, it's effectively cooperative because chances are you're scheduled most of the time anyway.