Rail systems could and if you look for it, already do form gutters along the panel-to-panel seams to not drip water inside the contiguous array area.

Still cheaper than a metal roof intermediate layer, though. But in places like Germany overhead glass outside of some limited applications (notably greenhouses for farming) isn't allowed to drop on anyone's head just from a sharp pebble impacting and cracking a glass layer. Sadly the regulations are just as strict for a home carport/garage-without-door as for a majestic train station hall. So aside from the worse cooling it's in practice cheaper to just use a corrugated sheet metal layer to catch rain and falling glass, with the structural support for both unified.