The problem is that we are negotiating from a position of ignorance. You don't know the lowest price they will sell it to you for, you don't know the lowest price you can get from competitors, and so on. Let's say I decide I will pay $25K for the car, sit there for 3 hours, and get that price. How do I know that if I presented a competing offer I might get it for $24K? I don't, and thus I leave money on the table.
Because when you don't have that info, the dealer performs a simple calculus - how likely are you to walk away after 3 hours of haggling over $1000 that you didn't know was negotiable. The answer is probably pretty small, on average, and thus it is worth it to them to not go rock bottom on the price.
If I can pay a service to get that rock bottom price (where price + service charge < my negotiated price) why would I not want to use it?
Because when you don't have that info, the dealer performs a simple calculus - how likely are you to walk away after 3 hours of haggling over $1000 that you didn't know was negotiable. The answer is probably pretty small, on average, and thus it is worth it to them to not go rock bottom on the price.
If I can pay a service to get that rock bottom price (where price + service charge < my negotiated price) why would I not want to use it?