OK, how about this reason....
don't start bad habits....eventually, one day, your snake will be big enough to reach the top without question. Only he's not going to tell you when that day comes, he's just gonna make a break for it.....
so, its better to just get into the good habit of only having the lid off when you are right there in the room, as opposed to leaving the lid off and maybe getting distracted and leaving it off for longer than you meant to, or having your snake make a fast break while your back is turned...
:eatpointe
(i just really like that smiley)