WASHINGTON'S criminal justice system, from its patrol officers to its courts, is among the nation's statistical leaders in arrests, convictions and incarcerations. The region's job market is also better than that of many other metropolitan areas. Why then are the city's illicit drug markets still thriving? Why is the murder rate still rising at a record pace?

Peter Reuter, Robert MacCoun and other writers of a new report by the Rand Corp. have provided some unhappy but plausible answers. Among the city's poorest residents, particularly young adults, the drug trade is a fixture in the world they inhabit, and it is seen as offering economic opportunities superior to anything else available. For those relatively few dealers surveyed who sold drugs daily, a net monthly income of $3,600 was possible. More than eight of 10 teenagers surveyed said they knew a friend who sold drugs, and more than seven of 10 said drugs were sold at their schools. One of every six black males (age 18 in 1985) who were a part of the report's analysis had been charged with selling drugs before he turned 21.

On its own, the availability of regular employment was not a deterrent to dealing drugs: some two-thirds of those who sold on a regular or an infrequent basis also held down (relatively low-paying) jobs. Drug dealing, at as much as $30 an hour, greatly supplemented their income. That drug dealing was much more dangerous than the most hazardous of legal professions was, again, by itself not much of a deterrent.

Among suburbanites, there has always been the sense that this was the city's problem. In reality, some 42 percent of those arrested in the city for drug possession during the period studied were not D.C. residents.

All this helps explain why larger police forces, more prosecutors and more prison space have had so little impact on the city's drug problem. It also explains why easy access to jobs hasn't stanched the desire to sell drugs.

That such a large part of the city's drug trade involves nonresident buyers suggests that more law-enforcement emphasis should be directed against those users. That so many young people here enter the drug trade as sellers and not as drug abusers underlines the need to pound home this message: Drug dealing seldom offers long periods of steady earnings. More often it results in permanent loss of most opportunities to find honest employment. More than any other form of ''work,'' it leads to incarceration and death.