Which is why you have zoning which is what people complain about. In addition, it's a lot easier in general to find a relatively undeveloped area and build high-rise condos there than increase the height of multiple blocks of an existing neighborhood by a few stories.
There are probably areas in the Bay where you could build some sort of tech worker arcology if you really wanted to do so and spend enough money. But I doubt people would really want to live there.
