Because with a skillet on high heat, it really doesn't take long to sear a steak and still keep it pretty rare. It also depends on the kind of pan you're using. A cheap pan will have a different heat distribution than a cast iron skillet.

Yes, you have to clean the pan. This is precisely why I use a torch to sear my meat. Even with the few times I use a pan, I prefer it because the reduced time on the pan means less oils splattering around my stove top, and I do think that the pan is still an easier clean this way. But that can all be avoided by using a good torch.