Because the family who hired the illegal guide can be kicked out and banned right alongside the guide. If they ban you, they're not refunding you your ticket.
I bet the guides don't mention that.
IMO, Disney should just do this once or twice (ban an entire group, guides & clients). If they do this and draw publicity to it via media outlets, a larger percentage people will still stop hiring these guides if they themselves run the risk of getting kicked out.
I think it's inevitable anyway. At WDW, they are going to soon change GAC rules. They're going do away with regular FPs. At DLR, they're enforcing FP return times. Won't be surprised if they start cracking down on other stuff.
There's the "illegal" word again. I have still not seen anyone provide proof that paid guides are "illegal". If you're jumping to that conclusion based on the assumption that the guide is using an AP, I can see that, but at least say that and not some blanket statement as if all paid guides are against Disney's policies. According the the lawsuit referenced above, it sounds like Disney does not have a problem with paid tour guides. What they have a problem with is misuse of APs, improper use of GACs, trademark infringement, and impersonating a Disney employee.
As for the conclusion drawn that a family hiring an "illegal" guide could be kicked out of the park and banned along with the guide, that is pure speculation and quite far fetched. I guess anyone can be kicked out at any time for any reason, but you're really reaching with that one. As much as you dream of it happening, it will never happen.
What gets me are the true issues brought up to date are misuse of Disney instruments (APs, trademarks, GACs, FPs, & images), yet so many have jumped to the conclusion that all paid tour guides are "illegal" or banned from Disney property, yet it appears Disney's own testimony is contrary to that conclusion.