In the leadup to societal collapse (you are here!), land ownership matters more than anything. It is immune to inflation. To a lesser extent, stocks matter. It's not like billionaires are sitting on a mountain of cold hard cash.
In the aftermath of societal collapse, control of ~~law enforcement~~ private militias matters. If you can feed and house a militia, then you can control access to farmlands, roads, all kinds of resources. Control of those resources will allow you to support your militia and provide sufficient coercion for people to "willingly" join that militia. The only tricky part here is the transitional phase, and honestly, there's probably enough cultural inertia that this will not be much of a problem at all.
See: feudalism. It is the wet dream of every ultra-rich piece of shit.
Most of the world is highly dependent on long-distance transport for the necessities of life, including food. Look at any major American city. None of them are anywhere close to self-sustaining. Self-sustainability is something America has not only ignored, but actively avoided and prevented in the design of its cities in favor of the "efficiency" of factory farms.
The best time to eat the rich was yesterday. The second-best time is today. Billionaires are an existential threat to society.
Same reason they don't today, generally: they are reliant on their jobs for their own personal safety and that of their families. Destroying the system that sustains them (even just barely) might not be in their immediate self-interest. They are disconnected from their peers (and those who would be their peers). Any direct action would be met with immediate hostility by the majority of the militia, and the best they could hope for is a volatile power vacuum.
See: prisoner's dilemma.
This does not rely on the rank-and-file enforcers to be particularly malicious people, only for them to have no clear and safe alternative.
If we're being perfectly honest, most of us are in similar situations today. I am fully aware that my tax dollars fund oppression all over the world, yet I still prefer to pay my taxes than go to prison. Realistically, I'm not going to stop participating in society, because it would hurt me immensely and it would help no one on its own. But I'm not kidding myself either; I am part of a corrupt system.
Real, lasting change requires organization and synchronicity. My choices as an individual are severely limited.