I don't know, maybe cuz it's also a Fruit named after a God,
And almost 70% of Japanese are Buddhists
I'm not gonna assume it's not that Strong, just because people wanna underrate Sengoku cuz they wanna ride Akainu's Dick instead.
Sengoku is literally named after MC of Manga that Oda basically worships, his Animal is literally a Goat, who was even used in Cover Volume to foreshadow the Future Yonko, He is as Legendary as Garp and represents the ceiling for Marine Career, even his Justice is "Reigning" Justice
In other words, Sengoku isn't just a Fleet Admiral, He doesn't follow Absolute Justice or any Justice shared by others, He Reigned like it was a Kingdom, if Roger was Pirate King, then Sengoku was Marine King (With WB/Garp being their Rivals who refused to compete for that Throne)
Actually, Roger's "Divine" Sword-Style & Sengoku's "Buddha" means They are more like Pirate God & Marine God.
So aside from Mihawk's Sword-Style & Dragon's DFs which are still reserved for longest time, i don't put any Power above Roger's & Sengoku's
Buddha is Buddha, it represents Peak Humanity (According to their Beliefs ofc).
People just Underrate it & ignore it's Significance because they wanna Underrate Sengoku for their Agendas.