The main purpose of airport security is not to prevent people from blowing up planes (although that’s an important secondary purpose) — it’s to prevent hijackings. Read about hijackings in the 60s before airport security; they were extremely common and disruptive.

Yes and no. Airport security showed up after US hijackings had already peaked, based on timing it seems the Federal Air Marshal Service service may have had a larger impact. The airport screenings are really an outgrowth of a Supreme Court case making such in depth screenings legal.

Also it was relatively frequent but not that serious in the late 1960’s. The first US fatality from aircraft high jacking didn’t occur until 1970.

Attacking airports has also happened multiple times including a 2002 incident in LA and even quite deadly attacks: https://en.wikipedia.org/wiki/Lod_Airport_massacre

why is a hijacking worse?

You can fly the plane into a building and kill thousands of people (source: 9/11)

As opposed to killing hundreds.

9/11 probably killed millions of people if you consider the destabilizing effect on geopolitics.

But can you really? The pilot doors are now locked from the inside and impenetrable.

That's a fair question, but I was answering why hijacking is worse, which implies that a hijacking can happen.

I'm not saying that the current TSA theater is the most efficient way to stop hijackings.