Manila by comparison is 71,263/km2 in it’s core. https://en.m.wikipedia.org/wiki/Manila
It's not dense relative to other types of metro areas, but I think it's probably fairly dense for single level housing in an industrialized nation.
My point wasn't to imply that Southern California was very densely housed, but that there may not be a huge amount of unutilized land left to build high density housing one, at least not enough to really change the dynamics. And once the conversation shifts from just what to build and where to also who to displace to build it, it's an much harder conversation.
I could be entirely wrong, I don't live in LA, but my impression every time I visit is that there's not a lot of unused land, even if the land that is used isn't used efficiently (population density wise). If that's true, it's not just a NIMBY situation (even if it might have traditionally been one), as it's also a "don't displace historical group X that's been living in this area for decades" situation (whether entirely accurate or not).
PS: It’s a question of underutilization, where it would be economically profitable to take each of these buildings down and replace them with more useful structures.
Therefore the riverfront should be low density huts forever I guess.
Out of respect for property rights though, please don't make it illegal for your neighbour to do this to their home should they choose.
It may have started as NIMBY, but I think natural market forces eventually reinforce it to the degree that NIMBY isn't necessarily the largest factor at play at this point.