They successfully tricked you into thinking that their prices are set by supply and demand. They're probably not the only company fooling you either. For just one other example, literal tons of unsold/unworn, perfectly wearable and desirable clothing gets shipped overseas, burned, or thrown into the ocean or landfills. The excess supply of clothing is so vast that it's now a form of pollution (https://www.nationalgeographic.com/environment/article/chile...). If clothing prices were set by supply and demand they'd be paying us to take the clothes off the rack. Supply and demand are a good explanation of economics for grade schoolers, but it doesn't explain the world we live in.
The idea that you can defeat greedy corporations just by treating customers better is a fiction. There's always more money to be made by screwing over everyone at every opportunity, and there's no shortage of greedy people willing to do exactly that. It's why we need the kinds of laws, regulations, and enforcement that even the playing field and allow ethical companies to thrive.
You can't jump into the middle of a rigged game where the referees have been bought off and expect to win by following all of the rules. You have to stop the cheaters and the cheating first to even have a chance.
I have no idea what the fair market price of a t-shirt would be in a world where no one had to compete with the practices of the current industry. I do know that it'd be worth every penny. The price being paid now in environmental harm, inequality, and human suffering is way too high.
I’m kinda surprised you never tried to do this considering how easy it is and how much your comments seem to suggest that you care. Possibly we’re talking past one another and you wouldn’t find any clothing companies that meet your ethical standards.