And I think it's partly because they
do do a lot of research, so they get some of the research prestige. With the number of full-time researchers and publication output they have, the research side of Google would rank as one of the larger institutes if it were standalone. See e.g.:
http://research.google.com/pubs/papers.htmlI think the same is true prestige-wise of other research-heavy companies. Bell Labs was prestigious; being at Microsoft Research is prestigious; etc.