maybe because they are done?

What do you consider early?  The deadline is less than a day away.   If the game is done, why take the chance of something coming up and not being able to upload at all?

you don’t get extra points for submitting early

the people are submitting when their game is ready, because this is the best way to make sure that the game is submitted before judging starts

I mean "because they are done" is a fine answer but...
When you go to the submissions page, are submissions listed in the order they were submitted?

Are you going to see the first submissions, first?

I don't actually know if the ordering changes based on anything else, but that could be a reason. Get seen, get more people to play your game. (I'm also totally unfamiliar with how voting works)

Edit: Or, hell, just in the hopes of getting some playtesting?
I've commented on a couple of the submitted games here and some of it was about when a feature doesn't work for me.
Which I'm hoping might be helpful.