For becoming the PK, you need the best CREW... And need to collect all 4 Poneglyphs. In this process, 2/3 yonkos may team up against you, like it is happening in Wano rn. Marines may hunt you down. You just can't deal with the Yonko crew or marines by yourself. You have to recruit finest crewmates and partner, like Zoro. That's why it's tougher to reach.
While, to become WSS, you don't need to become a pirate. You could also become a marine, or join any Yonko crew, or just train by yourself. You don't need any Poneglyph.
When the time comes, he also will. Oda has saved the best for the end.
Yeah I knew all that already, but all that stuff does not take ANYTHING away from their powerlvl. It's not like they had to commit to some kind of trade, as in either a crew + status or individual power, so that you become something like a Yonk or the PK. That's what's so faulty with that kind of categorical thinking.
Even though you need all that extra stuff besides individual power for that specific aim, you are still going to have to rely on tremendous individual power. You cannot exclude that from the PK title, it's practically impossible and was never portrayed as such, far in contrary Roger has insane portrayal and so does his equal.
Just take a look at Primebeard. Others are lucky if they can match his CoO, they are even luckier if they can holde a candle to his monstrous CoC and dang, he still got a bonafide S-class devil fruit on top. He has all that, that you named above and overcame all that , that you named above and still got that mind blowing individual power.
It's just a fact: Even silver medalist PK contenders- Yonks- are already top notch stuff, if you belong to them, so to be one of the greatest/strongest, you already have anything to ask for. You don't have to be stronger than them in order to be the WSS basically, cause it's a more isolated title.