In a divot with flap of earth covering the ball?

In order to deal with the kind of situation described by the OP.
Or, more commonly, so that a good drive is not penalised if it ends up in a deep divot.
(Plenty of people have asked why relief from a divot isn't allowed, and the usual response is "how do you know it's a divot?". If instead you just allow preferred lies any time, whether it's a divot is irrelevant).

OK, fair point

Rule 25-2 already deals with that (and at any time of the year, FWIW)

If you're saying that you're more likely to take a divot on soft ground in the winter, then allowing the ball to be moved out of a divot in the winter so that another one can be made next to it is hardly protecting the course, is it?

I'm not trying to be difficult - I just genuinely want to know the logical reason why preferred lies aren't allowed all the time. Someone like Colin or Rulefan or Duncan must know the reason.

EDIT: Oh, I see Colin has indeed responded while I was typing this. So the basic reason is that it would change the game. OK, I accept that.