Probably because once you send the CV they have enough (!) data to make a decision on whether you would be good fit, but with a two line email they can either disregard it or ask for more.
That's probably it. Plus there is an added benefit of a reverse filter. I don't want to work with any company that will, at the least, exchange a couple of emails with me. Any company who disqualifies me from just looking at my CV is not a good fit. Team chemistry for me is very important. I don't want to work in a place full of incompatible personalities. Not fun.
Most of the time filters are "all of", not "any of". A lot of companies may disqualify you if either you lack the necessary skills/talent or if you're a bad cultural fit. Disqualifying based on your CV just saves you time in this case - if they aren't going to hire you anyway, it's better if they let you know quickly rather than drag things out.
Plenty of places with good teams may have poor hiring practices.
Experience has shown me that poor hiring practices == poor team (in general).
How can a place get a good team if their hiring practice is poor?
Hiring practices aren't a constant. They may have changed their practices for the worse.
> I don't want to work with any company that will, at the least, exchange a couple of emails with me.