The paradox is that the person most likely to do extra work is the person you want to hear from least. If someone felt the need to pay (for example) $5,000 to talk to me, it's probably because they don't feel they have a chance of getting my attention otherwise. I'd also never pay someone 5 grand if I'm the one offering them something.

A Captcha isn't 5 grand, but I think the same principle applies. If I'm putting a request for proposal to 10 vendors, I'll probably disqualify the one that makes me verify that I'm a human. It's not like I'm short of qualified companies.