Initially the no. 1 reason for me was peer pressure. It was simply unacceptable to use something other than Google in tech circles at some point. It was like having an @aol.com email address.
If your users are that passionate, you're doing something right.
After I gave it a go for a while, the other reasons you list made me stay.
This is purely subjective, but Google is the only one that links to the Netlib 'real' LAPACK, straight to the file/documentation in question. The others have mixtures of Java packages and other examples...
Basically, but you're getting the opinion of a random person on the internet :). The third result is the 'real' one (the top two are unit tests for the named function.
If your users are that passionate, you're doing something right.
After I gave it a go for a while, the other reasons you list made me stay.