Perhaps it's because there's difference between voting *for* one party and voting *against* the other one. Unfortunately it's a subtlety that's lost at the ballot box, because they both appear in the same way with the same weight.
But before there can be any serious discussion about this you have to accept that, at the very least, there *might* be a reasoned case to support Labour, or at least to not actually support the Conservatives. And then there are local factors as well that come into play.
Finally, despite the narrowing of the polls, Labour aren't going to win -- or at least, probably aren't. If they do it would be a surprise for the ages -- and perhaps my delight at May's utter bells-up wouldn't last all that long -- but short of the shockiest of shocks, it's more a question of limiting the scale of victory than overturning the result. I don't want to see the Tories swept to power on a majority so large as to be essentially unshakeable for a decade or more -- hell, even a couple of Tory supporters don't want that, because they recognise the value of a strong opposition capable of holding the government to account. Whether or not we *actually* have that, Labour are the only party that could fill that role.
But the threat of Labour as you see it shouldn't even be an issue anyway. Whatever the polls say, and whatever the outcome ends up being, it shouldn't even be remotely this uncertain what will happen next week. May should have walked it. She's signally failed. That is *her* failure, and the failure of the Tory party as a whole.
Actually, if it comes to that, it's the failure of the Tory party for the last three elections. How didn't they win in 2010? In 2015 they were lucky in their enemies as well, if it comes to that -- the Lib Dems massacred from both sides, for being either too Tory or not Tory enough. And this election, they had a huge head start.
Look to May's failures, rather than the electorate's apparent unintelligence. She doesn't answer questions nearly as well as you seem to think, and has come across -- has presented herself -- very poorly indeed.