The title is not what's importance as it's just aesthetic, it's the act and influence (
over an area) that is key here.
The word King is just a title, but it means someone who has influence over a Kingdom; why wouldn't such a figure have existed in the past? that's similar to believing any form of leadership or authority only just came into existence.
900 years have gone since the Void Century, there would certainly have been Pirates wanting to claim territory as theirs or in fact, did own territory. Even gangs or small bands of thieves, have their own territory dependant on the extent of their notoriety.
It's a natural desire for Pirates aspire to be wealthy, feared & respected, free to do as they pleased and be treated as kings; that's exactly what an Emperor is; greed and selfishness are ubiquitous tendencies to those who naturally have aspirations of grandeur.
Very odd for this to become the norm only after Roger died even though he was never an Emperor, it's more likely that these things existed even before Roger's time.
Also, there's evidence that becoming Pirate King (
conquering the Grand Line) was an act attempted by Pirates before Roger became Pirate King -
it was said that people believed conquering the Grand Line was impossible. For there to be a Grand Line to conquer, it'd imply that there were territories too back then. Another point to put out is, if people considered conquering the Grand Line 'impossible', it suggests that people attempted to do so; can't claim something to be impossible without first knowing what it is.
Digressing a bit, how did Blackbeard take over Whitebeard's territory?
by conquering it; Becoming Pirate King and an Emperor both require conquering; what is the difference between an Emperor and Pirate King? An Emperor conquers a portion of the Grand Line and the Pirate King conquers the entire Grand Line. It'd make more sense that if people were aspiring to Conquer the Grand Line, then they'd start out with portions meaning, if people claimed Pirate King an impossible feat, the concept of Emperors must have been in existence too.
In conclusion, the question that should be asked is,
why wouldn't a powerful influential Pirate not own territory? is it that out of the ordinary to harbour such desires in the 900 years until Roger's death?