But why aren't you told until the last possible moment?

It does seem insanely stupid. But there is a raft of viable excuses.

IT IS GENUINELY BELIEVED
Often the programmers do not work at the software company. They may, in fact, be in a different country. So when the wily PR bod is telling you the game will be finished in three days, that's because it's what the programmers told them. In turn, the programmers may be expecting exactly that, but a terrible bug is found, or the premises are burgled, or the lead programmer vanishes, or something.

All these things, of course, have really happened.

IT IS A CALCULATED GAMBLE
More than anything else in the world - a larger car, say, or some ethics - the wily PR bod wants you to cover their game. Comprehensively cover it, in fact. On the cover. They don't want you to put someone else's game* on those pages. So as long as there's a chance the game will arrive, they'll keep you going without quite appreciating the stupendous risk. Or caring.

This, too, has happened. A number of times.

IT IS AN ERROR
They're thinking of the PC version, perhaps.

Oh yes.

IT IS A LIE
Because they hate you.

But anyway.