The reasons why may not be obvious if you're not a manager or haven't run a large organization before, but the simple truth is, "employees" are not a magically ethical bunch of people simply by virtue of being the underdog and poor and downtrodden and being all fired and stuff. You do not want a significant number of employees running around who are either uncertain they are going to be fired for a significant period of time, and even worse, you don't want a whole bunch of employees running around who have a pretty good idea they're going to be fired in a couple of days, because some fraction of them are going to take advantage of their access to wreck things up for the company. Wrecking things with technical access, wrecking relationships with companies... there's hardly anyone in a position in a company who couldn't do quite a bit of damage if they know they're out the door anyhow. All you have to do is be sure to keep it under the threshold where it's worth getting the legal system involved. (e.g., more "wreck relationships with customers" and keep the damage difficult to quantify and less "set the place on fire during peak business hours").
If a company told me they were going to fire me in two weeks, the worst risk the company would be taking is that I might just decide that they can't fire me in two weeks because I quit. I wouldn't do any other deliberate harm on the way out. Perhaps that's most people.
It certainly isn't all people, though.
Maybe it would be nice if we could all get along well enough to see being fired in some period of time as just business and not worth lashing out. Then again, maybe your sympathies lie with the employees lashing out. But in that case, the company has no choice but to get you out the door with as little warning as possible, to defend both itself as an abstract entity and all the rest of the employees.