This is a common, but very flawed misunderstanding of the situation in Israel, by analogizing it to 9/11. (Many Israelis were pushing this narrative at the start of this, btw, but that doesn't make it true.)
This wasn't just a terror attack, like 9/11 was. It was an invasion. Thousands of armed militants breached the border and invaded Israel, then killed more than a thousand Israelis. The militants were inside Israel for a few days after. In addition to the ground invasion, Hamas shot rockets into Israel, and has continues to shoot rockets at Israel this entire time.
This wasn't a terror attack. It was an invasion and a declaration of war.
In addition to all of that, for the first few weeks after this invasion, it was very unclear whether this would lead to a wider, joint attack by other forces as well. It is very hard for people in the US, and for most Westerners, to really understand what it's like to be afraid of losing a war. Israel probably would've survived even if this war turned into a combined attack by not just Hamas, but by Hezbollah and possibly even others. (Who, btw, are also shooting at Israel every day, just not doing it enough to really be considered another combatant.)
The response to 9/11 was maybe an overreaction. But 9/11 was not a real threat to the US , and this was fairly clear. This is an entirely different situation - there is a sworn enemy of Israel on its borders, proving they have the capability to kill thousands of citizens, and claiming they're going to do this over and over. That's a situation that a country can't live with, which is why Israel's reaction is not an overreaction - it's what any country on earth would do.
Notes:
1. This doesn't mean that everything Israel does is ok - there are different ways of prosecuting a war, Israel could still be doing it in a very immoral way and be consistent with everything I said above (I don't think that's true, but it's orthogonal to my point in this comment).
2. This comment also doesn't say anything about whether or not Hamas is "justified" in invading Israel, nor does it claim that "history started on October 7th" or anything silly like that. Of course there's context to this whole situation. (It should be needless to say but I'll say it anyway - there is no excuse for Hamas's deliberate targeting of civilians, and no excuse for the slaughter they committed. Still, that's also orthogonal to the point I'm making in this comment.)