I'm not sure. I think roofs are triangular mostly to make water/snow slide down, while arcs are architecturally stronger, i.e. they can support more weight.

So hexagons are better than triangles, and dodecahedron would be even better, etc. but nature tends to simplicity.

Not when you're pressing in on them from all sides. Look at arches and chicken eggs.

And what do you get when you have lots of triangles packed together?