Not to say that legal action hasn't been tried before. In 2015, a comedian named Robert Alexander Kaseberg filed a lawsuit against Conan O'Brien over monologue jokes that bore a striking resemblance to five of Kaseberg's tweets. The two parties eventually agreed to settle the case out of court. Conan adamantly maintains that he and his writing staff did not steal any material from Kaseberg or anyone else for that matter... he simply wanted to "forgo a potentially farcical and expensive jury trial in federal court over five jokes that don't even make sense anymore."
Comedians really don't have any industry watchdogs other than themselves. Musicians have organizations like BMI, ASCAP, and SESAC in their corner to handle their licensing, copyrights, and royalties for their creations. Film and television writers have the WGA. What do comedians have? The honor system... which does us absolutely no good when we're dealing with a talentless hack with no honor whatsoever.
Wanna know how easy it is to make tiny changes to something and present that idea in a suspiciously similar context where the original author has zero legal recourse? All of the famous examples of joke theft in the previous entry are in the exact same order as they're listed in the Wikipedia article for "Joke Theft". In the eyes of the law, rephrasing it in my own words means the original author has no practical case against me in court. But ethically speaking, that was a major dick move that would get my lazy ass fired from any self-respecting publication. My only saving grace is pointing out that I did it on purpose to prove a point. So there! I've become the monster I swore to hate! Are you happy now?
As comedians, it's frustrating to spend so much time and energy crafting and refining a joke, only to see someone else try to pass it off as their own. It's even more infuriating to find out that person has used that joke to become more successful than us. But to also know there's pretty much nothing we can do about it aside from stomping our feet and shouting "BUT THAT WAS MINE!" just makes us want to get stabby.