I have a theory that another reason the stock market and property values are growing so fast is that they're basically the only things that the upper middle class can invest in, so most of the money that well off people save just goes to pumping up the valuation of a couple different asset types that haven't actually changed in their fundamentals that much.

Upper middle class people can't really do venture capital or private equity (well, they can if they get accredited, but these also require a lot more knowledge and time), bond yields and interest rates are so low, and our retirement system essentially requires chasing inflation-beating yields to work, so they have no choice but to participate in bubbles.