Why did treaty cruisers default to 8-inch guns?

Could the United States have armed its treaty cruisers with 7-inch guns?

Yes, but the 8-inch treaty maximum became the competitive standard that navies felt compelled to match.

Nothing in the treaty prevented a navy from using 7-inch cruiser guns. Designers chose 8-inch weapons because that was the permitted maximum and other navies were doing the same. A legal ceiling became a competitive design target rather than merely an upper boundary. Arms-control categories can standardise fleets around the largest allowed capability.