Because it's hard, that's why!
It's hard keeping up with the changes.
The new stuff, fine, that's hard too, but it's interesting, and the docs for that have got a guaranteed audience.
But all those changes? Something minor affecting how the product looks or behaves, and suddenly the docs need changing... who knows where? ...maybe everywhere!
Or worse, some name gets changed, not affecting the product at all, just the documentation!
Worse still, some internal name gets changed, internal to the documentation itself, not visible to anyone except the doc writer who has to fix the resulting errors.
Ah, it could be worse...
...the developers could be in charge of the docs.