Because a mix of globalization, technological financialization, and perverse incentives.
The markets have become increasingly global to the point that there is never a shortage of investors or investment capital. However, profitable, low or no risk investment opportunities do not grow on trees nearly as plentifully. Due to an overabundant supply of investors with a high demand to seek profitable low or no risk investment opportunities to park their wealth for growth via interest, and a much lower supply of profitable low or no risk investment opportunities—the interest rates to park your wealth must fall. And they have fallen quite steadily and predictably since their peak in the 1970s and 1980s... which nicely corresponds with the technological globalization and financialization of markets and marks their triumph over inflation (why inflation has not returned).
There is another reason for inflated asset prices besides cheap interest rates. Many assets are also priced to interest rates in an inverted fashion (for bonds, that is exactly the formula how they work and are priced). Just before the recession, stock buy-backs were no longer prohibited and since the great recession and the ultra low interest rate environment coupled with stock buy-backs, stocks have become a slightly more risky alternative to bonds and stock prices became just as inflated as bonds and have more or less stayed that way. You see, when you have a combination of all those factors coupled with the ability to buy back your stock, stocks are now able to compete almost directly with bonds for investors and stock prices inflate accordingly.
Because globalization, technology, and financialization happen much faster than markets and regulation can respond, and there are vested interests by investors (corporations, banks, Wall Street, etc.) to maintain the status quo via lobbying the politicians and regulators... the situation is unlikely to resolve itself via some self-regulating efficient market hypothesis.