I'm not sure I would call those teams "great". They were about average as far as NBA Finals teams go IMO.
2016 - Cavs went 57-25 (0.695) in the regular season. They would have probably lost 4-1 to the Warriors were it not for the Draymond nut punch.
2017 - Cavs went 51-31 (0.622) in the regular season. Made it to the finals and lost 4-1 to the Warriors.
Regarding Giannis, I just don't think he makes others around him better. It is more about his style of play than anything else. He also doesn't seem to have the ultra-elite basketball IQ that guys like LBJ, Durant, and MJ have. He doesn't seem to have a sense of the moment. Maybe that will come.
I'm surprised that people always seem to rate Giannis above Kawhi Leonard. Kawhi can absolutely dominate a game on the defensive end... and then assassinate you on the offensive end. By the time Kawhi was 26 yrs old, he had won TWO NBA DPoY awards, had won a title, had a Finals MVP, and had 2 1st Team NBA selections. When Kawhi was 24-26, he was shooting 40.7% 3PT, 49.4% FG and 87.8% FT.