Same reason that Congress has not said to ex-congress members “you can't use your influence to help train up lobbyists and if we find out about it, that's your ass.” That reason would be … money and power for their “retirement” years.

Probably the same reason the US is powerless to say "cartels can't sell traffic drugs or humans into the US or it's their ass"

Because the military can’t tell people no longer in the military what they can and can’t do?

I mean, Congress could pass laws, but then you’d need to define a cartel, prove it in court, prove they were training, etc.