Two different scenarios and not acknowledging that is being obtuse. There wasn't a boss level threat after Goatku on earth when Vegeta had to use his ape form.
Meanwhile a Kai with more stature than King Kai is telling Goatku that Majin Buu is the worst thing yet. Because Goatku didn't listen the last time about Frieza and how powerful he was, he had to find out and also because he didn't want his friends to die. His best wasn't enough but he made sure ever since turning SSJ that he would always have a leg up on his opponents. Through ability, skill, planning or transformation, Goatku has always been able to win.
No surprise from Vegeta, letting Cell attain perfection and not having another level to go to in fighting him. He got bodied. Beat Goatku in the same way Gohan did to Trunks and had to kill himself to get rid of the shame of being beaten by pink play dough. Shame those 7 years he never leveled up in the mentality department.
Chess not checkers
Always think you're the strongest in the room when you can't see past the depth of your forehead.
Vegeta stans