Because they can.
These are all automated, and most of them have 6 or 8 easily identifiable patterns. Remove the capchka, filter the data on input, and if it's found, immediately exit with a "no email was sent" response. Many say this is unworthy; just proceed as if the email was sent. But the problem with this is they think it worked so they, and more, will keep trying. The above approach has proven (in my experience) that after a time, they give up. Take away the candy and the malcontents will stop trying to steal it. :-)
However, I can offer no help in getting a stock script to do this for you, it will require some custom programming.
There's only one reason spam continues: it works. So if not you, someone else.
Try millions . . . this is the whole deal, it's a game of numbers. P****ing off 2000 users is worth it if you get one sale, this is their philosophy. Generally they only have to find one "brand" of captcha, crack it, then seek out any sites that use that "brand." It's the same thing with forum or shopping cart vulnerabilities.