Phone/voice calls are on their way out anyway. Nobody has time to stop everything and talk on the phone anymore. ;) With a text they can finish what they are doing and give your text the proper response. Phone calls cannot be taken in public, especially when they are of a sensitive nature. People have to talk in private. If someone is making themselves available for say 9-12 hours a day, can it really be expected that they sit in private, ready to field a call at any moment?
Texting is private no matter where you are (as long as no one is looking over your shoulder). If you go out to get something to eat, go to the gym or drugstore, you can answer a text. A 'sensitive' voice call on the other hand can be overheard. Even the caller can be heard so it is a very strong possibility that the whole conversation can be overheard. Is that what clients want?
Everyone, not just Sp's, are choosing texting over voice calls. It is the way the world is going. Just search the internet for the opinions and observances of many people regarding voice calls...and even voicemail for that matter.
And it's like Jessica says about incalls. You pay to rent a space, then why throw that money away just so you can incur further costs to do an outcall?
The cost of outcalls for an independent are extreme and no client wants to pay to cover all the costs associated. And I really don't think any girls are willing to do everyone who wants to see them, delivered right to their door, a favour just for fun ;)
Outcalls are more easily suited for models/sp's who do not have or cannot acquire an incall location. They have the setup to deal with outcalls that is more efficient than the occasional outcall an independent/incall provider would have.