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The fabric can't always be rotated 90 degrees. Some T-shirts body portion is made from tubes of fabric. The ones with seams on the sides have the possiblity, but that highly depends on the weave of the t-shirt material itself. The material is knit in a way that is stretchy, and turning the pattern pieces (the only logical way to turn the fabric 90 degrees) can make the shirt not stretch the right directions when worn and/or force changes in the garment itself.

I'm not sure that it would.. the next sentence says that the shirt expands when worn and 2x more in the chest than the length.. that says to me that its not the weave, its just being stretched more in the chest direction so the length never recovers from its shrinking which becomes cumulative.

I think this is because the shirt is stretched horizontally just from being worn but obviously there's no forces stretching the shirt vertically because any force that would stretch it vertically just raises the bottom instead