Because to this date, it's the worst terror attack on US soil, it killed thousands of people and it was the direct cause to change our policy on terrorism.
9/11 changed this country drastically for better and worse, in more ways than one. I don't think there's anything wrong with remembering an important day in US history.
^this is why they to teach history, to explain why things today why they are that what and how they got that way. If you have knowledge of the past you use that information for your future decisions.
Just saying that is has no meaning, significance, or relevance to your life as well taking things for granted is negligent and ignorant in my opinion. Such people are usually responsible for wars, dictatorships, and disasters.