If you're a practicing physician and move to another state, you, in fact, probably do not have to retake licensure exams. Almost all states either have reciprocity agreements with at least a few other states, or will grant a license by endorsement to a physician who's licensed in another state and has practiced for a certain number of years. See
http://www.visalaw.com/wp-content/uploads/2014/10/physicianc...Board exams are the same as MCATs in the sense that you take them once and forget about it.
For lawyers, the situation isn't so rosy (which is to be expected, because law varies a lot per state), but there is quite a lot of reciprocity out there, and some states will offer an "attorney's exam" in lieu of a full bar exam. And, for some areas of law that are strictly federal, admission to the federal bar can be sufficient to practice in any state. It is not as hard as you make it out to be.