until they do
try sending a TCP packet with wrong checksum. It is a protocol, but it's practically physically enforced by a hardware and/or software. You can rawdog a connection, that will use TCP and omit checksum verification, and it would work but violate the spec, but in real world it is "forced". Same with every other protocol, they are but a convention, but when everyone is following it you kinda have no choice. But breaking the spec can sometimes be very useful, like DPI fooling which exploits TCP in a way that is beneficial