Because they're not trying to make money from the shares increasing in value sustainably; they're trying to make money in other ways. A couple of examples:

* Buying some shares, creating a bubble that causes those shares to increase in value non-sustainably, then selling the shares before the bubble pops;

* Getting compensated in stock options with a low exercise price, taking short-term actions that cause the share price to rise non-sustainably, exercising the options and selling the stock at a higher share price, then using a golden parachute to leave the company, letting others pay the price for the non-sustainable actions.