How else would airlines get little kids wide eyed at the vast array of lights and buttons and give them a branded pin and hat to win their hearts at an early age?

I think you're the only one who is baffled about an event that hasn't happened despite the doors being open on every single flight for 22 years.

Why is that a problem? A hijacker getting into the cockpit at that point can't really do all that much. The wheels are still blocked, and no tug is going to come along and push you back from the gate. There are likely still fuel lines and other umbilicals connected to the plane, not to mention the jet bridge. A flight attendant or other airline employee could pop down into the avionics bay (there are often access points outside the cockpit and even external to the plane) and start disconnecting things, making it really hard to get that plane moving.

And even if you could get past that, no one at ATC is going to give you taxi, runway, or takeoff clearance. I guess you could just go anyway, and of course ATC will get other planes out of your way, since that's the safest thing to do, but in all the time it takes for you to taxi and take off, the nearest Air Force base will have a lot of time to scramble a fighter to take you out as soon as you're airborne... or perhaps even before you take off.

The plane isn't a 500mph missile at that point, and law enforcement still have easy access.

It's not overly difficult to handle APU and engine startup on a modern plane, but there's still a bunch of other obstacles to overcome, wheel chocks, etc. And you're presumably not going to have a compliant tug to push you back (you can technically push back yourself with reverse thrust, but that's a whole thing unto itself).