Quite simply because investors are often chasing after money only (ie profitability), not necessarily a company's productivity or product quality. And the quicker the profits the better (meaning short term is valued over long term).
Additionally, not all investors are savvy on what's going on with companies, so they decide to invest in ETFs (which mind you, is now the lion's share of investment activity). Now not all ETFs are the same basket of stocks. Some may have 15 stocks, whereas others may consist of 50. This is done so people can "choose their risk tolerance", which is calculated based on various things such as how diversified the fund is (50 stocks is more diversified than 15), what the market caps of the companies are, etc. Anyhow, the point is, choosing to invest in said investment vehicles, has little to do with investing based upon fundamentals, or rewarding productive companies, and more to do with managing a person's personal investment risk. The result? Companies are detached from fundamentals because, quite literally, the majority of investors are not investing based on fundamentals, but based on risk.
Additionally, not all investors are savvy on what's going on with companies, so they decide to invest in ETFs (which mind you, is now the lion's share of investment activity). Now not all ETFs are the same basket of stocks. Some may have 15 stocks, whereas others may consist of 50. This is done so people can "choose their risk tolerance", which is calculated based on various things such as how diversified the fund is (50 stocks is more diversified than 15), what the market caps of the companies are, etc. Anyhow, the point is, choosing to invest in said investment vehicles, has little to do with investing based upon fundamentals, or rewarding productive companies, and more to do with managing a person's personal investment risk. The result? Companies are detached from fundamentals because, quite literally, the majority of investors are not investing based on fundamentals, but based on risk.