I have been asked this many times, so decided to post it on the main feed:

On site interviews are rarely used in United States residency programs. They are time-consuming for the interviewing programs because of the large number of staff needed to coordinate them, including department tours, the interview process, and providing lunch. This increases the time spent on the process for the programs themselves.

On the other hand, the candidates must travel to each interview, and in most cases they are not contiguous one after another but scattered, and even if they are in the same city or region it is unlikely for the interviews to be scheduled simultaneously. The cost is also a big factor for the candidates (airfare, car rental, overnight accommodations).

Please remember the purpose of the interview is the following:

Assess your ability to interact in English with the staff.

Assess whether you are a lone wolf or a team player

Assess that you have no obvious psychiatric problems (this is real guys)

Assess your ability to handle conflict. (This is sometimes used - one of the interviewers will be the "bad cop" and try to anger or aggravate you, just to see how you handle stress)