Why is it that problems that are obviously due to lack of man-power in a project are more likely to cause people to call for forking the project into another on

Why is it that problems that are obviously due to lack of man-power in a project are more likely to cause people to call for forking the project into another one that also lacks man-power, instead of considering joining the development team to increase its man-power? The phenomenon fundamentally doesn't make sense to me.