Frankly, the problem is that, ultimately, they want to hire the best people. Most of the time, the best people are sourced from the top-tier schools.
On the other hand, in my experience, they pride themselves on finding great people from outside of that small circle. When they do find such a person, it's not uncommon for he or she to excel.
It's totally legit to ask you what your definition of the 'best' candidate is, because you made a claim: that the best people come from the top schools.
How are we to conclude that your argument has any worth whatsoever when you aren't able to even define the components of the claim you are making?
Google is fairly vocal about their efforts to apply a somewhat scientific approach to comparing their hiring decisions to the subsequent performance of the candidates they ultimately hire. Here is an article, titled "Google's Secret to Hiring the Best People": https://www.wired.com/2015/04/hire-like-google/
Here's an article about how Steve Jobs wanted to "Hire the Best" http://recruitloop.com/blog/steve-jobs-top-hiring-tip-hire-t...
My personal definition is "high probability of excelling at the work that is outlined in the job description."