There is a ton of economic pressure right now to "put money to work." Interest rates have been low for a long time and industry is, for the most part, cashed up already. This means prices get high.
There were obvious seeds sown internationally after the banking crash, but I think tech companies may have something to do with it too.
Think of Tesla, a car manufacturer, and how different that is from a Facebook or Amazon.
Musk had to sell all shares (holding onto voting rights) and raise as much debt as he could. Tesla is still capital constrained.
Meanwhile, Amazon, Google and even apple are headed towards $1trn valuations without cash problems at all.
These companies basically create massive book assets (and income streams) without requiring much capital, or being capital constrained.
The growth potential and the cash needs are just not in the same place. This part of the economy isn't growing out "investment." It's adding to the lake of corporate cash, but corporate cash doesn't isn't used to make or grow googles.
We saw the baby version of this when of was writing about founders starting to keep majority ownership.