They believe it gives them some kind of advantage in getting off the plane sooner.
That if they stand up, get their luggage down from the overhead, and wait for the door to open, that they somehow will get off the plane significantly faster.
There may also be some amount of "follow the herd" mentality as well. That seeing one (or a few) stand up, others think those standing individuals are going to "get a jump on them" and they then themselves stand. Ultimately leading to a majority of "stand and wait for door" individuals.
The reality is this is likely partially a "tragedy of the commons" situation. If only a very few stood up, then yes, they would likely get off sooner than the rest that remain seated (because they are "ready to go" the moment the door opens). But if almost everyone stands (and seemingly almost everyone does) then the ensuing jam to get down the aisle likely means everyone ends up waiting longer overall.