This is the most honest article that I've seen from a mainstream financial figure about why valuations are so high.
ZIRP (zero interest rate policy) means that money is near worthless to financial institutions that can run the carry trade on Treasuries, or other carry trades involving foreign exchange. VCs manage money for those guys, along with pensions and other enormous concentrations of capital.
What you want are equity stakes (control) of productive assets. The cash that you use for that is not that useful except to normal suckers like you and I who need to hand over a bunch of paper tickets every month to the landlord, the grocer, and the government. The guys who issue the tickets are generally above those concerns, but they still need to get a return on their capital.
I would rather own stock than a pile of green tickets. Stock is, again, just another kind of ticket, but they're more expensive tickets that grant you rights to a productive asset. It is a safe bet to dump your green tickets into speculation because, while the speculation may work out, you know for a fact that those tickets are going to depreciate.
You will not see interest rates go up if the Fed can help it, because to do so would provoke an immediate fiscal crisis. Bubbles result from rational allocations of funds based on a certain set of assumptions. It is a good assumption that the US government will not permit rates to rise as long as there is zero public appetite for a major reduction in government spending. Once that reality changes, the structures that rely upon that environmental state will either need to adapt or will fail.
Startup-land is a greenhouse arrangement that thrives so long as the temperature remains high. When the guy who owns the greenhouse cuts the power, most of those plants are gonna die. If you want to be resilient to that risk, you must leave the hothouse. However, the hothouse plants have a lot more access to capital in the near term, so it is rough to compete with them directly. That is the trouble with bubbles: you can't really escape their effects by being 'prudent,' because low interest rates make prudence imprudent.
In order to raise rates, the US would probably have to either confiscate a lot of assets (which would harm its international status as the cleanest dirty shirt), raise taxes, raise retirement ages, implement means testing on medicare + social security, and cut military pensions/VA expenses. There is no real solution that does not involve provoking some sort of major crisis, so it is much easier for everyone to keep the carnival going as long as possible until something snaps internationally. The federal government can't afford even a slight rise in rates without having an immediate cash flow crisis.