Possible reasons are they get enough punters with the photos they do use. They don't care about the photos they use. They aren't very bright. They haven't the get up and go to get better photos. They are too tight to pay for better photos.
I would say all these reasons, some of which are similar to reasons why many don't have their own websites or adverts outside of AW.
An early client told me my poorly lit attempts at selfies were not doing me justice, so I've spent time and money on professional photos which I've a good idea does contribute to my business. Established clients and punters comment on them, and use them to indicate the style of lingerie and clothing they'd like me to wear for their booking. I do supplement the professional photos with a few of my own but I still find it difficult to get decent shots, also there's a discretion issue for me which means I prefer to / it's easier and better to go have a shoot in a completely out of area neutral location. In recent years the photographers I've used are a often a 2 hour or so drive away, so I'm making a significant effort which I consider an investment.