A good analogy comes from evolutionary biology. These techniques evolved alongside the techniques police use to bust them. The police/dealer relationship is an example of the Red Queen Effect:

an evolutionary hypothesis which proposes that organisms must constantly adapt, evolve, and proliferate not merely to gain reproductive advantage, but also simply to survive while pitted against ever-evolving opposing organisms in an ever-changing environment.

The police bust dealers. Dealers discover what evidence the police need to get convictions, and adopt techniques to prevent the police gaining enough evidence to get a conviction. The guy with the money never touches the drugs, and vice versa. The guy in charge of the corner never touches either of them, leaving low level transactions to underage employees. The guys in charge of the crew have little to do with the day to day dealing. Everything goes through middlemen, making it very hard for the police to tie any of the big players to a crime. The drug crews that do this the best flourish, while crews that do it poorly go to jail.
As a result, the police must come up with new strategies to gather enough evidence for convictions. In season 1 of "The Wire", they start tapping phones. In later seasons, the dealers react by using disposable "burner" phones. The police react again. On it goes.