A few things: we also put lots of effort into preventing mass casualty attacks in public places around the world (except the US govt in primary schools, for unfortunate three letter political reasons that begin with N and end with A). We generally don't want people to blow up planes because they're super expensive (even if we discount the lives of the passengers). Terrorists prefer blowing up aircraft to other things. When planes crash, they often hit people on the ground. And so on...

Train and subway stations rarely have more than one or two armed officers if they have any at all.

Train tracks have shockingly little protection to prevent timed derailment sabotage. Buildings have little protection from being surrounded by accelerants and set on fire, etc.

There is a strange subset of things terrorists seem to want to do and society doesn’t do much to prevent the rest.

There have been more terrorist car bombings than plane bombings by several orders of magnitude.

There are lots of reasons people bomb cars. I would bet most of those organizations (1+ individuals) would prefer to blow up planes full of specific targets over specific targets.

Do they? Other than 9/11, I'm personally unaware of this ever happening. 20 seconds searching says there was an airplane that crashed into a DC bridge in the 80's killing 4 motorists, and a couple of similar incidents, but it seems rare.

https://en.m.wikipedia.org/wiki/Pan_Am_Flight_103

11 residents killed by falling debris.

https://en.wikipedia.org/wiki/Pacific_Southwest_Airlines_Fli...

Seems to me a lot of terrorist attacks are blowing up buildings or driving vehicles into crowds or bombs in crowds or stuff like that. Not a ton of blowing up planes in transit in the overall list of terrorist attacks.