I can't speak for everyone, but for me it comes down to the fact that you only get one chance to submit.
I have had a submittable entry for a couple of days now, but given a bit of extra time, I'm happily polishing the submission a bit more.
I've worked with other systems that permit multiple submissions, with only the last submission being used. I realize the system is probably setup the way it is for some technical reason that is ultimately beyond the control of anyone listening, so it's not big deal.
I just end up panicking when the button moves around on the page!