She says the money is nice but she hates to be touched by most people (I already knew this about her which was another, smaller reason that I was surprised when I figured things out).
Slightly different situation than your girlfriend, OP, but I was sexually abused for years as a kid (ages 6-10). A couple of years after it stopped, I started having sex with older guys even though I despised being touched. For me, it was an attempt to prove to myself that I could take control and that sex could be a positive. I needed to prove to myself that sex acts were optional, and that I had the power to both initiate and end sessions. Between the ages of 12 and 18, I slept with 100+ people. Only three of those were under 18 years old, and one of those three was my best friend/boyfriend of two years. The oldest were in their 40s (I was 17).
When you are having sex for reasons other than intimacy/physical pleasure (i.e. abuse, sex work), the emotions and physical sensations involved are different. For me, I learned to suppress my feelings... It was necessary, or I would've gone completely crazy. But the feelings were never completely gone, just buried. Even though by the time I was 7 or so, I could bite my lip and not scream or cry while being raped or sodomized, I wasn't able to completely escape. I stopped sitting on my parents' laps when they read me bedtime stories. I would try to be in my room or the bathroom when company left so I wouldn't have to give goodbye hugs. I still (20 years later) have nightmares about it all and FREAK OUT when people touch me in my sleep.
The reason I dislike being touched is that people assume that it's okay to invade my privacy and to make contact without asking first. I have major issues with control as a result of my history, so I react negatively to anyone or any situation in which I feel is attempting to dominate or control me. A casual pat on the shoulder or a hug hello from an acquaintance might be intended as friendly, but it is an invasion of my space and I struggle to not flinch every time it happens. I wonder if that's the reason your girlfriend is able to do sex work, despite her dislike of physical contact (obviously, for both she and I, it's different with people you care about)... She obviously CHOSE her clients, and it sounds like she had a lot of control over her situation. Because of that, she might have been able to cope with the physical side of things better. Total guess though.