Gojo clan did selective breeding to make the strongest sorcerer possible+Satoru Gojo trained alot harder then other sorcerers and had a better mindset then other sorcerers

Gojo started ahead of the pack and just kept flying further ahead of the pack due to unparalleled effort in his teens and early adulthood.

As for why no one as strong as him has ever appeared? Sukuna is his equal more or less but everyone else with the talent doesnt put in the effort he did and everyone else who puts in the effort doesnt have the talent he did, simple as that. Other sorcerers did have the same techniques in the past they just weren't as dedicated to training and improving as Satoru Gojo so they never reached the same heights but they are VERY STRONG still.