What everyone else has said. It's the quality. Pros spend a lot of time doing the right thing.
Dogs sit around a lot between sessions but I think that's part of the learning experience. Too many dogs at home are doing things that are fun to them on their own. At the pros or even when we are training them their fun is training. Waiting between sessions makes the session more important to them, in my opinion. Many times it's a good idea to crate a dog for hours before you go to train. If you come home from work ready to train and the dog has been chasing squirrels or playing with the kids all day your planned force to pile session doesn't have much appeal.
You can do a lot on your own, but a dog has only so much energy and attention span so if you're doing multiple dogs or you're with a group it's one thing, one dog on your own you're both waiting between sessions. If you're traveling a distance to train it's very hard.
I agree that a pro can easily do 3-4 times as much as someone can on their own in the same time period.. They are also good at evaluating what a dog needs and where it is and how far it can go in training at the higher levels.