Except it isn't true. Other companies still make rockets in SoCal. For example Areojet Rocketdyne is still headquartered in El Segundo (next to LAX). They made the main engine for the Space Shuttle and are currently still making it for the upcoming SLS.
So much modern aerospace technology historically comes from SoCal, and yet CA as a state doesn't seem to care about hanging on to world-changing industries like it.
Your example about Aerojet Rocketdyne is telling. They are contracting - I believe the Aerojet facilities in Sacramento are being divested if that's not already done. Boeing is divesting most of its Huntington Beach campus, and Rockwell in Downey is long-gone. A lot of Space Shuttle Main Engine/RS-25 work is done at their Stennis facilities or in Huntsville.
Aerospace in CA is not healthy.
The pork of the SLS program and the RS-25 program has been well document.