Fan Casting Chained by Fashion in Sort by Tropes

Simply put, characters who are impossibly powerful tend to be incredibly dangerous, so they get limited, sealed, or bound to imprison them (in this particular case it must have been with chains). Like that'll work. To show this, when the chains inevitably break because the character is just that powerful, they'll often choose to keep the locked manacles as reminders of their captivity (humility, rare) or displays of their indomitability (pride, common). Then, there's always cases where they either can't remove the manacles or don't care to.