You know when you look through your feed on facebook, and every single day you see that at least one of your friends has done something amazing, commendable or just interesting? That makes you feel that you aren't doing anything with your life, and that all your friends seem to be living more interesting and happier lives.
But stop and think for a moment. Approach it from the other side. If you have 365 friends, and you post one interesting post a year you are faring as well as the average of your friends. It just seems that everyone is doing more interesting things than you because you only notice the interesting things your friends post. You only see the highlights. Nobody posts a "today was boring - I did absolutely nothing" status.
HN is the same. Think a bit about it.
If 10.000 people see a comment thread then statistically someone will be knowledgeable about the subject, whether its the French Revolution or the finer nuances of L2 caching. The most knowledgeable ones will feel they have something to contribute and write a comment. The vast majority won't. On top of that the top comment is (supposedly) the most well-written, eloquent and knowledgeable, and can thus be argued to be the best 10.000 people can come up with.
If I were to start commenting on threads about L2 caches I would make an absolute fool of myself, and prove to everyone that I have no clue what I'm talking about. So I don't.