Who says they're "waiting"? I would think "work that long" would be a more appropriate way to put it, as merely waiting won't get a company to an exit no matter how long you wait.

Anyway, the ones that wanted a quick exit have generally already had them, or given up and gone to work for someone else. There have been five or six YC exits, so far, and all of them count as "early cheap" exits (though most of them are now "rich" by some definition of rich, they were still merely high six to low eight figure exits)...the companies that are still in business and haven't been acquired after two or three years obviously have bigger plans. In the past couple of months, I've talked to three YC companies that have had acquisition offers but have turned them down.