Because people that run companies are idiots. Well, maybe they’re not quite idiots, but they’ve got a business on the line and you don’t. It makes them overly cautious. If you don’t sling out shit on their timetable, you’ll get canned. Of course, even a single hire getting terminated will set back the company by way more than they bargain for, but good luck getting them to realize that.
Don’t be surprised if you get calls from your old employers a year down the line asking if you can go back to work for them. Those are always fun.