I'll turn your question around, why don't these hip companies relocate to the suburbs?
There's plenty of abandoned malls and shopping centers that could become offices, such as what Rackspace did.
Instead these companies locate downtown, where parking is crazy, crime is high, tons of crowds, and everything is expensive.
The suburb model has been done by other companies that aren't hip start ups or tech.
Maybe it's time for these businesses to relocate?