A lot of Silicon Valley's high-flyers are not yet great IPO candidates. Of those that could be taken public, many would face much lower valuations in the public markets, forcing them to raise less capital or sell more of themselves to raise equivalent amounts of capital to what they're raising in the private market.
As such, one could argue that the startups have been acting completely rationally by taking advantage of the willingness in the private markets to invest at exorbitant valuations. Sure, the late stage valuations are all engineered and companies will pay a hefty price if they can't deliver the returns they're increasingly having to promise late-stage investors, but I think a lot of these startups have made a calculated decision to raise as much capital as they can selling the least amount of equity, and worry about the consequences later.
It's not surprising the traditional VCs aren't thrilled with this.