I am no expert, but I am guessing it is because many startups are pre-profit or even pre-revenue. Whether you consider Twitter to still be a startup is another question. However, consider that until fairly recently, much effort had been devoted to scaling, quite possibly at the expense of new features and strategies to drive revenue.
Becuase it's only investors that have skin (money) in the game and so for them the valuation is more important than profit as they want to be able to exit at a far higher valuation than when they entered. Profits are only important to people working there or that own shares and are long term investors.
Because businesses by their nature are generally serving the middle class, but the money is concentrated among the top .01%.
This leads to big valuations and relatively low revenues. As wealth continues to concentrate at the top of society the trend will continue. This would not have happened 100 years ago when the top tax bracket was ~90%.
Money is worth less to those with wheelbarrows of it. Ownership matters more. Call it inflation if you want.
"This would not have happened 100 years ago when the top tax bracket was ~90%."
Not to be too pedantic, but 100 years ago was a great time to be a millionaire, the highest tax brackets were inconsequential, the tax for the top bracket of $500,000 was 7%. Rockefeller didn't have too much tax planning to worry about.
The high brackets came with the depression and later WWII.
