Economic reality
If its true that its that easy, then you wouldn't have to wait to see it, it would already be there.
For every new sp that pops into CL, there is another one that retires (new life, baby, school, not suited to the work, moves away, etc etc)
In the end, there is a finite number of positions to fill because there is a finite number of available clients. They either have to make a go of it full time, which costs a lot more than they might expect it to and is very economically uncertain, or they do it part-time, limiting their availability to potential clients. Often to the extent that they see very few people and don't even make much of an impact.
Plus, certain types of sps are in short supply and will not be replaced by any random female. Dominant, mature, experienced, white, etc cannot be replaced by the asian invasion, for one thing. There will often be a new supply of sps available for the purpose of "getting off", but there is and will always be a limited supply of sps who have skills beyond that.