Vegetation make people feel closer to nature. It is, after all, what most of us expect to see out in the wild. Most vegetation that is observed in forests and undeveloped areas really is natural. Much of the associated insects and wildlife are natural as well. Such flora and fauna know how to survive within their respective ecosystems. They can not rely on any unnatural intervention from anyone.
Naturalized exotic (non-native) species proliferate only because they are adapted to similar environmental conditions. A lack of pathogens that afflicted them within their natural ranges is a major advantage for most of them. Nonetheless, they are unnatural components of what is commonly considered to be nature. Most naturalized exotic species actually interfere significantly with nature.
Vegetation and associated wildlife that inhabits synthetic landscapes is very different from that which lives out in nature. Only some of the vegetation has potential to naturalize. Even less is native. Almost all of it is reliant on artificial intervention for survival, particularly irrigation. Associated wildlife is reliant on the survival of the reliant vegetation. Landscapes accommodate. Nature does not.
With few exceptions, landscapes that emulate nature are impractical. Landscapes within forests are some of those few exceptions that might need no more than what the forests provide. Even in such situations, combustible vegetation and structurally deficient trees should be cleared away from homes. In California, nature is innately combustible. It is messy and potentially dangerous too.
Most urban landscapes of California would still be dreadfully bleak if limited to natural components. Both San Jose and Los Angeles are naturally chaparral regions. They were formerly inhabited by sparsely dispersed trees on scrubby grasslands. Now, relatively abundant vegetation in both regions is more appealing, and improves urban lifestyles, but is nothing like what nature intended.
Nature is simply inadequate for what is expected of urban landscapes of California.