T-shirts are generally stretchy, so they don't need to be custom-made to your body size.
From personal experience: I can buy ready-to-wear shirts, but the armholes are slightly larger than necessary, and there's always excess fabric around the waste, which means it bunches up when tucked in. Some places have choices of regular/slim/extra-slim, but even extra-slim is a bit baggy for me.
When I wear a well-fitting shirt (either custom-made, or one where I've used my sewing machine to take in the sides), the shirt doesn't look better. _I_ look better.
From personal experience: I can buy ready-to-wear shirts, but the armholes are slightly larger than necessary, and there's always excess fabric around the waste, which means it bunches up when tucked in. Some places have choices of regular/slim/extra-slim, but even extra-slim is a bit baggy for me.
When I wear a well-fitting shirt (either custom-made, or one where I've used my sewing machine to take in the sides), the shirt doesn't look better. _I_ look better.