Ok, first you must understand what a studio offers and realise that most physical studios are in developing countries, mainly Eastern Europe and South America.
A studio offers a place (a room) for performers to work. They provide the computer, lighting and internet. Some studios provide training and translation services. The studio sets up the performers with the sites that they work on and collects all the payments from the sites and then pays the performers. For this work studios take between 30-60% of a performers earnings. Studios range from completely exploitive places to sort of legitimate places that somewhat look after and respect their performers. Some of the better studios provide healthcare and pay the taxes as required.
Why would anyone want to join...
A lot of performers that work in studios can’t afford the equipment and internet connection... it would not be uncommon for a performer in South American to not even have their own bank account. Often they live with family and don’t have a private space where to work from. For some of these performers even $300 a month is a considerable amount of money. A big thing is also that a lot of these performers are unaware of how to get paid from a cam site or even how to join.
For most western countries, I can’t see too many performers needing the services of a studio. If you buy second hand, you can get starting equipment reasonably cheaply. Most people have a reasonable internet connection and with just a little research you can find out how to join a site and get paid. Also most performers would at least have a private bedroom to work from... Making the business case for a studio in a more expensive city might also be hard.
Please note there is a lot of generalisation in my comments...