World government unleashed their own army of giant killer robots against Whitebeard, yet I have to see anybody copy them on that.

Plus one of Luffy's crewmembers is a cyborg and everybody knows he's a cyborg for a lot of time yet no other cyborgs yet besides that one shibukai that was used as the template for the killer robot army.

Also the whole previous arc was Sanji getting married to a daughter of Big Momma because Big Momma wanted all the super tech from Sanji's family, meaning she couldn't reverse-engineer it on her own.
Besides cyborgs not being that uncommon, then they are not that powerful unless build by Vegapunk.

And no, its likely far, far easier to just steal the Genma's super tech, than it is to try and develop it yourself.
Its also not organisations like Big Mom i would worry about reverse engineering Seastone bullets.
But the Genma 66, Vegapunk or Ceasar? yes easily.