Probably because there are about a million other more important injustices to work on first.
In the grand scheme of things, I'd argue that occupying a beach to stick it to the man isn't a great way to spend time.
An "Occupy Malibu" protest needn't suck resources from cancer research. A few hundred (thousand) people are going to be visiting beaches in the general area of Malibu anyway. A concerted effort by beach-goers to find and use the most egregiously obstructed public beaches (for instance, using this app) could be both satisfying and low-cost.