They’re both called “knitwear,” but a sweater and a knit T-shirt are built in completely different ways.
A sweater starts from yarn. The panels — front, back, sleeves — are each knitted from a single continuous thread, then joined together by linking the individual stitches one by one. The seam is nearly invisible. The inside lies flat.
A knit T-shirt starts from fabric. The knitted cloth is woven in rolls, then cut into panels and the raw edges are secured with an overlocker. You can see the overlocking stitches clearly on the inside.
One process starts with yarn. The other starts with cloth. Two different logics, two different cost structures, two different quality levels.
Next time you pick up a garment, flip it over and look at the inside seam. That tells you where it came from.