Some of them play bullet all night long 
why do GMs lose
GMs lose because they are drunk. Atleast Kasparov, who in a game with Karpov, missed not only a mate in 8, but also a mate in 1.
Some tournaments are long and stresful, so fatigue towards the end of a diffiult tournament can be a factor.
Another is the ability to play under the pressure of time control.
Lack of preparation for a certain line. The opponent has studied a certain line in detail.
why do GMs lose
Same reason your father has a 1 in 10 chance of being your biological father:
a stranger willing to pay 5 dollars.
they are humans, some time because are tired or maybe something are happening in their brain, we are humans and make mistakes, thats why they lose
if they are so good and then they should not lose.
Ok, so that's like me saying...
1. Why do you lose?
2. Why do I lose?
3. Why do NM's lose?
4. Why do IM's lose?
5. Why do GM's win?
6. Why do why do?
7. Why does everyone lose?
8. Are you a human?
9. Why?
gms lose because....
they know no one will sign there paychecks if its just constant draws?
---
and that brings up a half serious point. at higher levels, could (generally) two grandmasters play draw after draw? I mean obviously no Gm wants to lose- but do they deliberately take risks because they know if they don't it will just be draw after grinding draw.
(i might be asking a dumb question). if so sorry. its just THAT kind of thread.

if they are so good and then they should not lose.