Because chances are, if you are using a password that is in the list, it's because either it's an exceedingly common password (and you really shouldn't be using it) or you've used it before multiple times and are probably the reason it is in the list (because it was breached on another site).

From experience, most attacks we see now are credential stuffing attacks rather than pure brute force attacks using something like Sentry MBA, with a huge number of IP addresses (the last attack we saw was using over 6 million IP addresses). So throttling sign in attempts at the IP level is almost useless as is throttling at the email level, as the attacker can attempt at least 6 million known email/password combinations to see if those accounts exist on your site.

The only real defence against that is all your users using 2 factor, or creating a psuedo 2nd factor (email them if the attempt is from an unrecognised IP).

Edit: Of course the other helpful defence is to ensure your users aren't reusing passwords, which is where Pwned Passwords comes in.