Indeed, it's like people completely forget about such attempts in the form of the Berlin-wall or the border between the two Koreas.

Heavily militarized borders with everything technically possible defending them, regardless of inhumane it was/is, yet didn't stop people from attempting to cross and actually crossing.

Even the great wall of China isn't a good example because afaik that wasn't built to stop immigration, it was built to stop raiding parties.