First and foremost, a lot of us watched both play and we just believe he's better. He was a better scorer full stop. He was a better, more consistent defender over his entire career. He's neck and neck with Lebron when it comes to leading in the traditional per game stats, but he's better than Lebron in almost all of the per 48 advanced stats. He's got more accolades (with the exceptions of All Star and All NBA nominations that Lebron stacked up because of more years played). He won with an organically grown team and didn't have to ring chase. He didn't need a super team to win.
Honestly, the GOAT debate is kinda fake. When you actually examine it deeply, it's not really all that close. The only thing that Lebron has over MJ is longevity and nobody gave a shit about that until Lebron's fans started acting like it was the end all be all. Why is his greatness enhanced by him sticking around racking up bulk stats to no end. The prominence of this debate is mostly the result of propaganda (Klutch Mafia) and a lot of people in the discussion being too young to even have seen or remember MJ in action. It's fine to like Lebron more than MJ for any reason, but again, people have way more reasons for putting MJ over him. Only dummies like SAS run around yelling 6-0 like its the ultimate smoking gun.