You sorta have to make your own fun. For me it's just having funny interactions with other people, for others its gang wars. Find what you like to do. You'll also find that money leaves quickly.
For example, yesterday we kidnapped a group of jahova witnesses who were all named "Brother [name]" It was an interesting experience that ended in them killing a lot of people for being non believers.
Also where you said "I don't understand cartels...why are they so important to everyone" Well honestly they're not important, they generate money which you have to share with your whole gang. But that's not the point, the fun is taking and holding those cartels from all the other gangs.
You have to make your own fun, that's the point of RP servers.