Why don't (all) professional painters use paint sprayers?
The paint needs to be of appropriate consistency. If the paint deviates too much it could clog the sprayer.
overspray (paint landing where you don't want it to) is a big issue, avoiding that means spending a lot of time masking stuff off and/or building a tent around what is being painted. Depending on the job this can take longer than breaking out your brushes/rollers and doing it the old fashioned way while only masking the edges.
Sprayers need an air compressor, which needs power and can be quite loud. You also need to run the hose from the compressor to the spray gun. 
The amount of paint in the spray gun is limited and fairly likely not enough to last as long as a tray of paint or a small bucket that you can take up a ladder. Leading to more up and down movement making the tiring painter.
When people hire a professional painter they expect a minimal quality; uneven finish looks like very shoddy work and can lead to a lot of clients refusing to pay.