He was called that by the WG/newspaper/big news Morgan
There was no pirate meetings like in pirates of the Caribbean or anything where pirates determine that
Same with naming the one piece, both in the same chapter
The reality is so far oda didn't potray that every pirate by default wants the one piece and that makes you the greatest
Infact this is proven in this flashback, xebec is seen as greater pirate than dodger
Xebec did what ? Usual pirate shit and his world dominating plan
Roger did what he do what a one piece seeker would do, exploring around
And despite both not achieving their goal
It’s not that complicated it’s a shonen and a classic one
Roger was introduced to us as the Pirate King, as the one who started the story, the one who found the One piece (the title of the story)
and he is the goal of the MC (or the second main goal wherever)
Luffy is yelling « I want to be Pirate King » every time he can and he wants to find the ultimate treasure the One Piece and be the freest pirate on the sea
Roger is on top, no doubt about that
And yeah nobody elected Roger but : Garp glazed Roger, WB glazed Roger, Shiki glazed Roger, Kaido glazed Roger, the whole WG glazed Roger giving him that worldwide execution and wanting to eradicate his offsprings etc etc
Even Xebec respected Roger
Roger is that guy
it is completely Oda fault that Roger character execution is quite bad and we have that kind of debate but the truth is Roger is written to be that guy, even if the writinh is bad