I'm 25, and I think I speak for a lot of folks my age when I saw that Google is the 'Bell Labs / IBM / whatever big influential company that makes a mark on you as a young adult" company of our time. Now I know that may not be the case now, but when I was deciding that being in technology was what I truly wanted to do (teenage years), Google was coming out with Gmail, and was the 'key' to the internet with search, and bought YouTube/Android, and was everywhere, so I really saw them as a pillar of modern day technology, and to a sense I still do. So whenever I get a call from a recruiter I can't help but be excited. I know their hiring practices are not perfect, but they are still a company who I look towards with bright eyes.*
* Albeit less bright as time goes on.
It's premium branding, plain and simple, and the fact that Google employees have to justify the effort and time they put in to get hired in the first place partially explains why Google is rated so highly by current/past employees.
I'd be really curious to learn how much Google spends in man-hours for each hire in comparison to other tech companies.