If you look at things through a Western lens, then (yeah) it looks terrible.
However:
1. The reason these girls are primarily here is to make $$ (for any number of reasons), not to have a good time. A lot of them have some combination of debts and family to take care of back home.
2. Many speak little or no English & are unfamiliar with the culture. They also typically don't have cars or drive. So traveling or being out-on-the-town is not a lot of fun for them. Many have flat-out old me they consider America "boring", probably for this reason. Others may be a little afraid of American in general. So, compared to the outside world, inside the spa may seem much more comfortable to them.
3. Some of them work very hard in China, so they are used to long hours. Others are just not that sophisticated and may have spent a lot of time at home doing not much anyway.
4. It is not all "work". Many of these girls spend the majority of their day sleeping, watching TV, playing games, surfing the net, cooking and talking. Especially if they work in a place where they get 3 or fewer customers per day (which is not unusual). Some of them complain more about cleaning chores they have to do around the spa than servicing customers.