It also mostly doesn't work, and even if it does work it's terribly expensive and time consuming enough to scare people off.

Go on, make a derivative of Mickey Mouse and sell it. See how it goes. Similar enough to be "compatible" (whatever that would mean in the animated cartoon space) but distinct enough not to run afoul of Disney lawyers. Then come back and tell us.

there are event exact measurements to take into account, for visual art, music etc. 'what is legally not stealing'.

Art, however, is a little different than code. code is a thing, but it also produces things.

It weirds me out there is a measure of code similarity but not a measure of if code is semantically the same. for example implementing a protocol could be done in many ways, but ultimately whats talked between clients/servers on the network is the same. so it's semantically the same despite being totally different code.