Frankly, airplanes suck. You spend endless time in security lines, get packed in like sardines during the flight, and have stringent restrictions on what kind of luggage you can take, how large it is, and how heavy.
People will probably end up "packed like sardines" on HSR as well, as HSR will inevitably have to be made to be economical.
Other countries that deal with terrorism operate train systems without people groping your privates. Yet another problem with airplanes is a tiny bomb can cause explosive decompression. That's not a worry with trains - you're not going to kill 300 people with a bomb you can hide in your underwear.
We have the TSA because people are afraid, and we don't travel by plane enough to care.
Because in places where HSR does currently exist, there are no significant security lines.
Trains have fundamental safety advantages compared to planes, so the payoff of an attack is less.