Why the default gallery border?

This is more a general question than a support ticket. Feel free to move it if there’s a better place for it.

Why the default 2px grey border on gallery images?

It’s easy enough to get rid of with !important; and I understand the styles for gallery positioning being in the core, but it seems unnecessary for borders/colors like this to be built in and only possible to override by force. Was just curious about the reasoning.