History.
So, it's World War I, right, and American soldiers were sweating their tushies off in the trenches wearing their hot grandma-knit woollen uniforms. Meanwhile, the Europeans were all like "eh, Americain monsieur, looke at us being cleane and breezy in our light cotton undershirts, trés comfortable indeed, ha ha."
In true American fashion, they copied the idea and just gave it a different name: "T-shirt," referring to the shape, of course. By WWII the U.S. army had made it their standard undergarment. After the war, people started wearing T-shirts without a shirt covering it, and people were shocked. Then, Rebel Without a Cause came along, and James Dean saved the day for all of us once more.
The hippies, bless 'em, went on to tie-dye the shirts and print actual words on them, and ever since the sixties T-shirts have been a sort of medium for self-expression, displaying any text conceivable, as well as artwork or even photography. Too bad a lot of them express nothing but lies, dirty lies.
