If you've ever changed tires on a fwd car and kept it in gear, you will notice you can torque the front tires far harder than the rear, as the front will be held by the engine and the rear wheels can spin surprisingly easily by your own power even when held by the hand brake.

Also depending on weather and temperature the handbrake can fail.

I’ve never seen anyone successfully start an engine from stand-still with it in gear (though I’ve seen people try). You get it rolling in neutral and only then engage the gear to start the engine without a starter or battery.

And parking brakes by themselves are some of the most fail prone systems in the cars.

Your engine will lose compression over time so it's plausible that it'll roll a bit without the brake engaged.