I can see why Beerus never felt bothered to master UI; until Goku came along in his life he had no one from within his universe that could really challenge him, so what’d be the point? Especially if he was already one of the strongest if not the strongest GoD.
I hope for Beerus’ sake though that he does start god/UI training again soon, being alarmed at how Goku has come along now with UI... or the writers can just have him let Goku pass him up without a fight or comeback like the Goku dickriders want.