I usually think they are either trying to get you to click on it as if it is a need to claim the draw, since at times, you may be wainting for it to pop up yourself. I also think some of them think that people are stupid. Perhaps most ananoying is that, they want to distract you, P!$$ you off, so that you can't concentrate on finishing them off when you have a decisive advantage.
I wish they had a force forfeit , after being abused in won position button....
I always encounterd player who offers me draw when I almost defeat them, sometimes they make me wait and leave the game without resigning. I wish I don't encounterd such a player anymore