This stems from the need to understand all the little aspects of trial practice, not the least of which is how to resolve cases by pushing the parties to settlement.  Beyond settlement (which accounts for 90-some percent of all civil case dispositions), there are highly developed fields of arcana - the law of evidence, for starters - that only really get into a lawyer's head by actually doing it again and again.  Then, there's knowing when an argument is bogus, weak, diversionary and when not, knowledge gained primarily by trying to make that argument yourself (or watching someone else do it).

I'm not saying you have to have an apartment in the Courthouse, but rather that you have to know which side of the courtroom you sit on.  The judges who make good judges without a lot of courtroom experience are primarily appellate judges, for whom riding herd on a room full of trial lawyers' egos is not a job requirement.  A lot of law professors could make great appellate judges - but not trial judges.