It's the usual comparative-advantage thing, though. Five seconds of your brain reading something is essentially worthless to you (unless someone is paying you to read this comment), but of value to Google. The resource behind the CAPTCHA is of minimal value to Google (what are they going to do with an account at startup-of-the-month), but of more value to you. The transaction is mutually beneficial.

The value Google gets from the thing they want, and the value you get from the thing you want, aren't particularly comparable.