If rent wasnt super high, where would the person that happily rents an apartment for $4k a month actually live? Since his apartment wouldn't be available as someone paying $900 a month would still be in it.
Many Californians acknowledge that there needs to be more housing built, but many of them are also NIMBYs who don't want multi-family housing built in their areas, because it'd lower their personal property value if there was more housing available.