>> Stainless steel is just such a terrible choice for a rocket this large.

Honest question, but what do they usually use?

Aluminum alloy and carbon fiber, because both have high strength to weight ratios. Steel is denser, so its specific strength isn't as high. But it retains strength at both lower & higher temperatures, and it's much cheaper. It's also cheaper/quicker to modify a steel design, as welding is easier than aluminum (at least for low carbon steels used in Starship). And if the same structure were made of carbon fiber, it would likely have to be fabricated from scratch.

I think choosing steel has helped Starship, as it allowed the ship to survive reentry and land in the ocean despite heat shield failures. Had the ship been made of aluminum, I'm certain earlier missions would have broken up when the aluminum heated up and lost its strength.