When Ninja is playing, you are watching a dude play a training/pick-up game with randoms. No loser penalties, no prize pool, no stakes. He plays 6h+ nonstop, literally nothing matters (including the small tournaments that he plays in). Win or lose the same Fortnite matches will continue forever.
When you are watching the NBA game, you are watching many people compete for money/career, survival in the competition and fame. If you follow one team, you have like 2h of airtime in one week.
The argument can boil down to literally "you are watching something on the screen, how is it different", and then there is no difference in the content preparations, production value, importance, stakes, continuity.
Both are quite unproductive activities, but come on, watching your NBA team is not the same as watching your daily streamer. If you value his personality or skills more than the sports broadcast, then hurray, enjoy superior content for you.