The prostitutes are far more willing with sex, quickly, so you don't waste much time getting them there, they don't give you problems when you've had enough of them and pay them off (quite unlike a normal lady), and there are a variety of other lovely beauties out there for your next adventure. For what you pay per night, it's pretty cheap when compared to the real costs (both time and money) on an alternative, serious relationship, and you avoid the emotional pitfalls of something more serious. Even if you keep a prostitute for a few days or weeks and enter a grey area relationship, she'll usually be out of your way with her next distraction -- another farang man in her normal environment...
For those who want a "good" Thai girlfriend, I can advise you about this, but before doing so you'd better think about whether this is really what you want in the long run. "Be careful what you wish for, because you might just get it!"
Once you get a good Thai lady serious about you, especially after physical intimacy, if you change your mind then they will usually be a lot more difficult to break with than a Western lady. They are not like prostitutes who are already accustomed to this (but who may yell and scream for a few minutes nevertheless, but still leave after you pay them off). You cannot pay off most mainstream Thai ladies. They wouldn't accept it, and they won't give up easily. It may be more traumatic to the average Thai lady than the typical foreign woman.
Relationships consume a lot of time. Businessmen and independent guys often just don't have the time, or don't want the entanglements. For these guys, the occasional weekend outing in the bars and music is a time & cost efficient release.