Adding SWEs to a late project makes it even later.

Here's how:
- initial: adding new people pulls people of away from doing the work
- ramp up: new people need early feedback cycles before their work is valuable
- ongoing: more people means more communication overhead as you continue

This last one really kills you. As you add people, the number of communication channels grows quadratically.

All those communication channels take time, and that's by definition the thing you're running out of.