An arrest doesn’t always equal to charges. You can theoretically be arrested 30 times and never stand in front of a judge once if the DA consistently chooses not to file charges or waits until a later date to file all the charges together (as may be the case here).

So best guess is that it’s a combination of the cops deciding to pick up the same people anytime anything bad happens in that neighborhood, whether there’s evidence they’re involved or not, and the prosecutors knowing they had evidence to file charges for many, many crimes but wanting to make sure all their ducks are in a row before filing every possible charge against all the alleged gang members all at once, which would mean potentially catching and releasing some of them a few times for various suspected crimes.