Simple really, it looks good all those girls
But in my opinion it has massive disadvantages which outweigh any of those benefits of looking like a 'busy agency.
The main one is if you see a fit girl who is never working, you hold off booking anyone waiting for that fit girl to be working. It is a case of the problem with agencies is you never know who will be working that next week. So you think if I hold off "xxx" maybe is working next week. And then when she is not you think, maybe next week.
And thus all that happens is you book nobody waiting for that one girl who looks could to come on and give a shift.
If she was removed you would just see who ever else is on the lists.
The next problem is, a load of girls being on the books makes it harder to check them all out. And thus you may miss someone you like. Amour currently have 66 girls on their books. And Diamonds currently have 47 girls on their books. Both of those is far too many to hunt and locate girls who would take your fancy. If both agencies removed the 'dead wood' so to speak (as in dead wood meaning girls who have not been working for weeks and likely wont be working again for a few more weeks) it would make it easier to look through the remaining girls.
And, and I would say this is a biggie, it would work as good PR advertising when a girl does come back as they can chuck it in their news page. "Paige has decided to come back after a 2 month holiday". "Autumn has returned after a 3 week break". That announcement works as good advertising which is not there by simply having the girl show as available again.
It is the role of agencies to actively promote all the girls on their books so its odd why they don't do this to be honest.