I was told by an SP that many girls don't like wearing their normal street clothes when working as that is an intrusion into their personal life, which for a variety of reasons they prefer to keep separate. Seems to me that's a pretty stand up reason.
An SP has got from the time she opens the door to the time you hand over the cash to impress you enough to convince you to stay, a big part of that is her appearance, well it is for me. The problem is that underwear, as in bra & panties is not always the best look for a woman, some of them need some additional help, which is where some nice clothes comes in, most people, even blokes look better clothed to some extent.