In California, state law holds that stealing merchandise worth $950 or less is just a misdemeanor, which means that law enforcement probably won’t bother to investigate, and if they do, prosecutors will let it go. Most packages are probably under the $950 threshold, so there is no enforcement. Why bother investigating and arresting when the DA won't prosecute and they'll be let out immediately?
Although to be fair, wasn't / isn't a lot of this "why bother" actually a political protest and retaliation against that DA and the citizenry that elected them? Not even doing something else with their time, but not doing much? And to be fair again, the joke in San Francisco is that "not doing anything" is a safer alternative for some of the population. Safer than "doing something else". This is messed up every which way.