However, i do not understand streetgangs like the blood and the crips. They have no real structure or leaders.
Yeah, wrong. They have a pretty organized structure similar to the mobsters and old-time criminals the culture tends to look up to. Why do you think every "hood" rapper has to make references to Scarface -- that is, both Al Capone and Tony Montana. It's because they go into crime looking for the same thing and they use a similar structure that they learned from history. There's grunts, there's muscle, there's bosses. It varies from gang to gang but it's generally more organized the more profitable their business is. Also, on that note...
When the smallest earner "earns" something, no one else would care. The money he earns is his alone, thus really eliminating the need for the gang in the first place.
Wrong again. The dealers or hustlers or movers of this illegal quantity or that... They all give upward, just like the mob example you're using. Sure they take money off the top but the bosses organize things and get the connections for drugs or guns, and the low-level people have to respect that by giving most of the money back upwards. The entire system is actually pretty f*cking organized despite it looking otherwise. Street gangs are dangerous because of this organization, and especially nowadays as cartels continue to supply to them, gangs are an issue for the entire region they are in.
They take in young people as grunts in the business with a promise of an easier life, or at least a more honorable one, or at least one that makes more money. And usually that promise is correct because these people they're recruiting feel like if they don't sling dope then they're going to end up flipping burgers or something for the rest of their life. These gangs feed on that anxiety that every young adult has. Compound that with the culture we have in America that glamorizes material wealth and showmanship over anything else, and you would be just as willing to get into your older brother's gang, if it meant making more money than your high school teacher at 17, getting laid, getting high, being the toughest motherf*cker on the block or at least friends with the toughest guy out there. It's a sense of community that they won't find elsewhere, and the reason why is because these gangs are organized just as much as any mafia. They are, in short, one in the same.