Because time matters. Look at elections (where there are huge incentives to exploit the fixed timescale), or look at fraudulent operations like pyramid schemes - eventually they get exposed, but if the originators are sufficiently disciplined to cash out and bail before that they may reap handsome profits. The incentives to spread false information are a function of the profits that can be obtained from doing so before the next decision cycle.

As a parallel example, look at the way the clock influences strategies in professional sports, with teams using timeouts and deliberate fouls to game the outcome of a match. At that point they're not playing ball, but rather a meta-game about time management. It's one reason I like baseball; the game runs for a defined number of plays (9 innings, each of which runs until 3 batters have been struck out), so while games can run very long, that sort of metagame doesn't really exist.