Basically extremely low chance of windfall, plus transaction costs. Most brokers charge you to place an order; even if the price is just a few pennies, if your chance of that order actually being filled is one in an extremely low number, you're still losing money on the deal. Additionally, "good till canceled" orders (which is what you want to use here) are time-limited on most exchanges, typically to 30 days. That means you need to spend mental bandwidth refreshing them every month, which for something that has an infinitesimal chance of paying out, usually isn't worth it. (I basically let my order lapse after a month because I forgot about it.) The orders exist because out of millions of market participants, somebody's going to place one, but it generally isn't cost-effective for that person to be you.

With regulated exchanges like the major stock ones, you also have to worry about your transactions being invalidated, eg. in the 2010 flash crash nobody who did this actually got paid out because the exchange said "Oops, software glitch" and canceled all transactions before settlement (with the stock market, you don't actually receive the stock until 3 days after you make the transaction).

In addition to the sibling comment, there's a non-trivial risk that, when the crash happens, it's because of a legitimate, non-local drop in value, and you're stuck holding something worthless.