Interest rates have been so low that nobody wants to invest in bonds or other interest-bearing funds. Where else are people with money going to invest? Once interest rates start coming up, the picture is going to change dramatically.
It doesn't seem as bad as it was in the late 90's, when investors were throwing money at anyone who could do something "on a computer." At least this time around, most of the companies with high valuations actually do something valuable, even if they don't yet know how to make money. Still, there are a lot of crazy stock prices out there.