AFAIK America has had plenty of shootings, and probably arson attacks too over that time period.
I agree on guns, but you can probably deal with that with much lower intensity security.
You're right that fortunately there aren't many cases of people causing fires inside airliners on purpose. But that doesn't mean it couldn't happen. When a single power bank can cause catastrophic results like this, I'm glad there's at least some monitoring of what people carry into the airplane in their bags.
In other words the TSA specifically does not seek yo prevent fires. The reason we don't have people setting fires on planes is because people don't want to do that. And if they did the TSA would be specifically useless in preventing it.