You'd have to build many more barriers and gates to stop that, which would be awkward for baggage systems, fuelling, and general maintenance.

The question is more - how hard is it to get airside without a pass?

I once went in through the main arrivals exit doors at Heathrow. I'd forgotten something so I went in, got it, came out again.

It was a stupid thing to do, but I wasn't arrested, stopped, or anythinged.

This was before 9/11 so I doubt you could do it today. But even so.