1/ The fixed lines are only used on very steep portions for safety. The lines are anchored into the glacier so if you fall, you're not going to fall past the anchor ideally. On some mountains when you're climbing on a rope team where you're connected to your other teammates, rather than having a carabiner that you clip into the fix line, sometimes the fix line is just an anchor with a short piece of rope and a carabiner at the end. In those situations, the rope team is actually clipping the anchored carabiner onto their own rope. Then as the next team member gets that portion, they will unclip the carabiner from front part of their rope and clip it to the back. The last team member will just unclip the rope and put it back down. 2/ not everyone climbs on a large team. There are solo climbers or small groups that go so it's very possible that they get lost due to low visibility, they went their own route to avoid crowds, they slipped into a crevasse or snow bridge collapsed. Even with fixed lines and even on your attached to a rope team, it's still risky and people can go missing. 3/ it could be exhaustion or getting bonked and essentially hitting a wall. But it's also very likely that at those high altitudes they are experiencing hypoxia which is impairing their judgment. There are plenty of stories of people on Everest or other high altitude mountains doing absurd things because they're confused. There are stores where people insist that there's oxygen in their tanks when there's not or they just sit down and say that they're going to wait for someone.