Maybe you've just been unlucky.
It doesn't make sense otherwise.
I usually pay as soon as I realise I have won. These days, with so many buy it now ads, that means pretty well on the spot.
The only reason I can think of for some buyers to be late might be if they forget they put in a bid or don't realise they have won.
Don't assume email mesages to them will necessarily reach them as I know in my case, my ebay email isn't my real, 'working' email, I don't check it.
It is possible for a bidder to win an auction even after they have been outbid. Happened to me once, with someone who withdrew a bid over a day later, so naturally I had thought I was out of the running and wasn't back on ebay till a couple of days after the auction ended. Got a shock, but paid straight away.
If i hadn't gone in though, I would not have realised in time and i could have ended up with a strike against my name, the ebay system is most unfair sometimes, the way it lets some buyers retract bids.
But I suspect the main reason you have had delayed payments is some buyers are a bit lazy or are quick to bid, more reluctant to pay.
As you say though, if you are going to pay, may as well pay within the time frame.
