This has been asked many times in the past and on many boards and is a good question.  I don't know the exact answer but I can take a guess.

The first thing you have to remember is that escorting is legal.  That is, hiring someone for companionship is 100% legal.  It's only the sex for money part that is illegal.  The smarter agencies will verify that you're not LE (as best as they can) before they will discuss with you any sex for pay. The really paranoid agencies won't even discuss it with you until you've seen a couple of their ladies.

The second thing is that, for the most part, LE has better things to do than to go after escort agencies.  LE will go after streetwalkers because people who live in the neighborhood will complain.  They'll go after escorts and agencies who use hotels a lot when the hotel staff complain.  But other than that, escort agencies don't usually bother anyone so LE pretty much leaves them alone... unless someone complains about them.  Often it's an ex employee who complains.

The things that will cause LE to go after the agencies when there's no complaint from a former employer or hotel staff: drugs, underage participants, failure to pay taxes, and high profile clients with enemies.

As I said, I don't have all the answers but this is my take on it based on what I've seen over the years, and from conversations with escorts, agencies, lawyers, and others.