Nearly half of all Mayo Medical School graduates stay at Mayo for their residency. Some people even choose Mayo Medical School specifically because they know they will have a better chance at getting a competitive Mayo residency such as Ortho or Neurosurgery.
First, the facts: Although I have not surveyed every medical school in the country, it is my distinct impression from the match lists I have seen, that at virtually every medical school with a major teaching center, a disproportionate number stay at their primary teaching affiliate. Furthermore, geography exerts a disproportionate influence at state schools (California students tend to stay in CA, NY students tend to stay in NY, Michigan students tend to stay in Michigan). This is not to say that people don't go, indeed from my informal look, only about 1/3 stay, but considering that there are over 200 major teaching centers around the hospital, having 1/3 stay in any one geographic region is pretty high.
You already know the system. You can get things done faster. The attendings already like you (they took you after all!). You know the city and have friends in the city you are at. Significant others already have a job (if non-medical SO), or often easier to get couples matched at your home center.
No excitement of somewhere new. You may miss out on a chance to learn something totally new that your medical school doesn't have (i.e. the subspecialties that were weak in your medical education, will remain weak during your residency training). You don't get to start with a clean slate (if you had it out with an attending or another resident in the past, you have to put up with them for the next 3-7 years).