Its from the same brand of clothing as the Hulk's purple pants.
Its from the same brand of clothing as the Hulk's purple pants.
______________________________________
"Always be yourself... unless you suck."
Fans asked Oda about the stretchable clothing years ago in an SBS. His response was something along the lines of "are you saying you WANT his clothes to rip?"
Basically there's no reasonable in-story explanation for it.