An airliner's external doors are designed such that they're held closed by internal pressure. Thus opening a door on the ground is pretty easy, which is convenient because that's the only place you should open them. But opening at cruise is extremely difficult.

Now - a strong person can do it when the plane is far enough up that it's very scary, after all some normal passenger airports are a few thousand feet above sea level and we want the doors to work there. But you won't open them at cruise.

All the normal doors on an aeroplane are fire doors so they can't be locked from the inside.