Sure, the same amount of stone could be used to construct several smaller churches. The US congress could also just rent a conference room at a nearby hotel if they chose. The Eiffel tower could supply iron for several kilometres of rail track, or maybe a small boat.

But building something extravagantly big has a signaling value all of its own: "see the glory of <whatever it is we constructed this for> and how much resources they command". You don't build a cathedral because it's more practical than a normal church for holding services and stuff, you build a cathedral to express the power of your religion and impress it on others.