They are either:
1. A fish
2. A stupid bot (incompetent programmer)
3. A corrupt bot (drunken programmer)
4. A trumpy bot (not accepting rules and limits)
5. A busy player, busy with other things like posting in a chess forum
6. A stupid player, not acknowledging the abyss
7. A childish player, waiting for the opponent to give up even if he is about to win
8. A bullheaded person not accepting his loss
...
surely there's more
Having played in 'the park' a bit in the last few weeks, I've noticed plenty of blitz players don't resign even after blundering a queen or blowing a position completely. They play on to make sure that their opponent doesn't make any more mistakes (which sometimes they get overconfident and they do). I saw a guy recently in a game act like he was owning the whole park, and he made a terrible move which lost instantly and he just got up and walked away. It happens.
So we are talking bullet/blitz games now??? I never resign in bullet