One of the major reasons that people use 3rd party sites to sell is that it's far easier to use someone else's format and templates to get a site live faster than to try to figure out how to do it on your own. Even though website programming has gotten a lot easier, it can still be a steep learning curve for someone who has never dealt with HTML or templates before. If they don't want to take the time to learn, many 3rd party sites are extremely user-friendly and have a working staff that can help people to get their stores up 
in record time.