Hmm? I've done interviews for both of my past two employers. The first one was a startup, but the second one is Google.
Do the big companies you work for not have people interview with employees in their particular role? It seems relatively common among my friends for them to be interviewing people just 1-2 years younger than them, either for internships or to fill the position they're about to be promoted out of.
A startup isn't a massive company and Google is notoriously atypical with transparency and inclusiveness with hiring. Even so, just conducting an interview is far from learning about the thought process behind the actual act of hiring.
Do the big companies you work for not have people interview with employees in their particular role? It seems relatively common among my friends for them to be interviewing people just 1-2 years younger than them, either for internships or to fill the position they're about to be promoted out of.