Major European cities have had centuries to develop, and were built in large part before automobiles existed, which necessitated density and walkability. Silicon Valley, by contrast, was largely agricultural or undeveloped even 50-60 years ago, and development of suburban housing represented a significant increase in density (and automobile transportation was cheap and fast then, so relatively low density housing made sense).

Now, we are left with the problem that the suburbs aren't going anywhere for the most part, due to lack of political will, and lack of a reasonable market-based solution -- single family homes are often in the $1-1.5 million range for ~1/8 acre lots in less desirable areas, and redeveloping even a relatively small subdivision would be extremely expensive, even if you could get everyone to sell (they wouldn't), and get permission from the city. Redevelopment is limited right now to the commercial corridors, such as El Camino Blvd, and existing commercial/industrial/business districts, all of which are undergoing pretty heavy redevelopment in much of Santa Clara County. Transportation infrastructure including both public transportation and public roads are also undergoing multi-billion dollar improvements, though some of the work may take a decade or more to complete.

source: I have lived in the Silicon Valley area for over 30 years -- its changing faster now than at any point I can remember.