True. Licenses do not protect you or your work directly, but they can only help you if you want to bring stuff in court or something, but unless the offender is a bit commercial company or something doing this on a big scale it will not be worth the trouble (not to mention that such companies can easily put themselves above the law, so you'll need a good lawyer too, and some publicity).
Now I do not know how the itch.io staff responds to clear license violations on smaller skills. For example, Scyndi's Creative Interpreter is a game engine I created myself and released under the terms of the GPL3. Makes you free to use my engine, even in commercial productions actually, but if you modify the original source code create a new game with it, sell it without releasing the modified source code, then we got a violation of the GPL3. Of course, I cannot prevent people doing that, but I take it the itch.io rules do ban that practice, right? That being said, it can still be handy to have a license included in your game.
When it comes to protecting yourself against copyright infringement in general. I was young in the 80s and 90s and have seen the countless creative ways in which game developers put in copy protections to prevent piracy. They all failed as document with the codes so you could print them, or even hackers managing to remove these protections were countless, so piracy happened any way and for legal users they were only a big source of irritation. Long story short, it was futile.