Cause there is a rivarly, that simple.
Oda went out of his way this time to make it clear to distracted readers once and for all: he could've given different individual bounties and Emperor status just to Luffy, instead he decoded to differentiate these 2 things:
- 3,000,000,000 bounty to each of the 3, cause they individually do rapresent the same threat/danger
- Emperor status to Luffy only cause of his crew and influence (Fishman Island, Dressrosa, Wano...)
I mean, couldn't be clearer than that at this point