Blind as a Bat

Why do people say "blind as a bat" if bats aren't blind?

It's bad enough that you're already insulting whoever it is you're calling "blind as a bat", but do you really have to go and insult bats as well?

So they can't see very well. Cut the poor creatures some slack. They already have big ears and leathery skin. I'm sure they already get enough crap from the rest of the animal world.