Because google can already identify you through your cookies etc. If you turn off tracking and cookies it makes you do it when you login every time.
Yes, I'm getting the recaptcha every time I re-login.
Ugh. Apparently this happens when HN is under attack. Not sure if the cause justifies the means in this case. For many HN'ers having their HN account associated with their other Google database entries or employment records would be a line they surely do not want to cross.