When you have a large volume of applicants and new hires (like Google does) then you can actually start to measure the correlation of the things you can assess in the interview vs long term job performance. Then you can make your decisions about who to hire based on this evidence rather than gut feel, subjectivity, biases or even CS trivia. It's much harder for a small company to do that, so at best they can end up copying the methods of the large ones and hope that their situation is similar enough.