Partly because insurance will cover it and partly because a lot of doctors haven't caught up on the new guidelines. A lot of this is newer knowledge based on more recent research and data, and with for-profit healthcare there's less incentive to let doctors know that certain exams and tests aren't needed and less incentive for the doctors not to do them since they're getting paid for it.

There are actually many other routine care things (for example prostate exams) that are also being questioned based on recent research but doctors are slower to catch up. The sad other side of this is that the uninsured end up without adequate care while those with insurance or money get over-screened. If we had a better system then care could be more equitable.

Personally I choose a family physician over a gyno because they have more time and are less patronizing. There are some women who don't know they have a choice because they assume only a gyno can do things like paps and birth control and others who are just more comfortable with a specialist. Many people in the US will just go to a specialist if their insurance doesn't require a referral or they're paying OOP whereas in other countries you have to see a PCP first.