It's so annoying, they just keep building up troops and won't kill your last man
Is there a reason/benefit these numpty/s gain from it?
There is no benefit for them other than knowing they're probably annoying you and definitely wasting your time. Maybe you did something in the game that pissed them off, or maybe they're just being a prick. If I was in a 100% hopeless situation and someone was toying with me like that, I would just quit. You'll save yourself the time/frustration and won't give them the satisfaction of annoying you.
I've had a few of those. Pretty pathetic people who get their jollies taunting you in an online game. Of course given the cheating, it's par for the course