I ask myself this question every time I knock on the door of a provider's 141 "office" or hotel room and wait the 5- 25 seconds(*) for her to open, particularly with a new one where I haven't seen her before and wonder what she will look like relative to any photos posted.
I never really have any original answers, and can only relate to what most others say:
want to see new faces and bodies - the genetic differences argument - men create millions of sperm every day and need to spread their "seeds", whereas women are born with finite (several hundred) eggs and stay with one person
they will do things my wife won't - even asking for some things may be damaging to the relationship, my wife is very traditional and conservative (and I like it this way), and this extends to sex
* two of my regulars never give me a chance to knock. At the appointed time they watch the monitors and as I approach they simply open the door.
Keep on mongering