Proprietary developers don't care because their code wasn't trained on. Is that a trick question?
They do care now that reverse engineering is possible.
Because the chance of someone auditing the actual source code is near zero, but for open source software it's near 100% that someone or, worse something aka an AI bot looking for a revenue stream, will have a look at your code. And EVEN IF you get caught stealing by whatever random incident (say, your code repository being 0wned) - it's a matter of a settlement payment. If you are not a company, but an individual person... well, just look at what happened to Aaron Swartz RIP.
And for the WINE/ReactOS/console homebrew folks, it's vital to be able to claim no code contributor ever got exposed, even indirectly, to proprietary code.
Your point about being caught is valid, but it depends on AIs producing identifiable chunks of code, and that is incompatible with your license. AI training is going to lean towards open source, because its available to train on.
I also think the widespread use of AI will mean accidental copying will become more acceptable. Even as it is, its going to be regarded as an accident.
As others said, for closed source software it's a lot harder to detect plagiarism.
And it's in general very hard to detect plagiarized code; there's some product that does it, but it of course has only a partial inventory of the existing open-source code.
I think that some big company does use those products, though.