I don't think the women that are kept until nearer the end see it as competing against 24 others .... because they're not. A LOT of those women are already gone, long gone. I think darned few of them come in the first night thinking they're going to be chosen "THE ONE" (though the occasional one that does usually makes for entertainment). It's when it's getting down to 6-8, and they presumably have at least a little "history" with Sean, that they start thinking, "Hey maybe this IS it." The odds still are not good, but as so many have said over the years, they have a good connection with the Bachelor or Bachelorette, and they don't know until its all over how much more the B had with their chosen one.
And as entertaining as we viewers may find it to mock and pick on the "contestants" -- if they stop coming, the fun is over. Cut them some slack.