One of the reasons is flow control. You want to be able to move a lot of water slowly until it’s where you want it. Many small inlets distributed along the surface is a simple and effective method. I’m sure achieving the design you’re speculating on is possible but it would be more complex and expensive for that design to reliably achieve similar goals/metrics.

Permeable pavements like pervious concrete and porous asphalt allow some of what you’re interested in without requiring a reinvention of the wheel.