Because the headline isn't part of the story, it's part of the index/ToC of the whole paper, so it's written by the staff that put the whole paper together.

The latency of decision cycle was too great in previous times. Even within digital workflows the length of time between knowledge of final space available on the physical page and when that space had to be filled with text was too short to haggle with a writer. It had to be performed by a smaller set of people right next to the people laying the type of not the people laying the type.

The story is worked on throughout the day/week. It goes through a number of copy editors and revisions. Its final publication date might be in limbo.

It's chosen for Wednesday in the budget meeting. It's 1A material. A cursory headline is written. Then a big news event happens: now it's 2A material. That means less space. The copy needs to be cut, the headline needs to be one line shorter, etc. This happens at, say, 9 pm and the journalist and editor in chief have been at home for hours.

Two reasons: the headline writers need to know exactly how many characters to use, as part of the prepress process, right in the layout room, while the journalists are out in the field, and 2) headline writing is its own skill (especially for tabloid headlines).