Coz he can't??
Chain >>>>>>>> in one piece
No matter how strong the person is........chain/shackles is the material that is always used to restrained or captured a person just like IRL.
No matter if you're Big mom or Ussop a shackles will restrained you
It's stupid but it's a fiction in the end, Oda can't explain either
Plus it's probably a sea stone, which is supposed to be among the hardest materials in one piece, i don't see anybody destroy it yet in canon stories.