When I first became interested in the Arduino and got myself a Uno. I couldn't understand why it never had pins underneath.
Essentially because you are expected to be able to mount it. Some
board must be at the bottom, and it makes some sense that it be the "master". Notwithstanding all the contrary comments, it is
intended that this be a functional product for "real world" applications. Commercial products do
use pin-and-socket connectors, otherwise they would not be manufactured.
How's this for a mod? Screw terminals on a Pro Mini.
I like that