I mean, "gathered the strongest" but all of the figher are acquaintances of the people that gather the fighters. For the last match: Takeshi & Nikita : Sayoko; Teru, Ainosuke, & Odamaki : Iwamuro.
And it's mentioned that Okouchi influence only goes in the main island. So it makes sense that there might be someone strong go undetected in small islands like Okinawa. And it's not like there are tons of them, Tobio's underlings are chumps compared to Kinji and Yosuke, who lost to Teru on a 2-on-1 fight.
For "reserve" the author still have secret organizations, another countries top fighters, and another country's secret organizations (and more).