Setback seatposts

Why do bike makers spec bikes with set back seat posts? Shouldn’t they just make the the seat angle slacker? That was the worse sentence ever on this forum. I will let it stand as is.

I understand using a setback seatpost during fitting but why spec it?

I’d say that the slacker the seat angle, the further rearward the rear wheel must be. Using a setback seat post is a workaround that gets your seat over the rear wheel, without pushing it too far back.

Geometry gurus, chime in here!

Rick