Holy knights and world government are afraid of luffy and other emperors
Nah: WG is scared of Shanks.
Shanks told the Gorosei a long time ago: "Hey, so, this kid stole the Nika Fruit from me. He's like a son to me, so, if you go after Luffy...
I will respond in full force."
That's why the WG only uses the Marines to go after Luffy: even with Robin's kidnapping, Cipher Pol, who takes orders from the Gorosei in many cases, made a deal with Robin to protect the Straw Hats.
This makes no sense: why wouldn't Cipher Pol just KILL the Pirate crew?
The only sensible answer is Shanks told the WG there would be war if they touched Luffy directly. Best they could do was go after Robin.
Look at what dying Whitebeard did to Marineford. Sengoku and Teach both wanted nothing to do with the dude who casually blocked Akainu.
Shanks is friends with A LOT of people. If the WG pushed him, he could decimate a lot of their forces. Like, Shanks ARRANGED Whitebeard's funeral. Shanks could gather Rayleigh + Scopper from Roger's crew + others, Giant Pirates, his own crew, and plenty of other individuals if needed. The WG does not want to fuck with Shanks.
The Gorosei trusted the Marines to handle Luffy, and... well, the man is lucky/loved by the Sun.
