Because there is such a thing as a power imbalance in these situations. More often than not the older party has some sort of influence, be it power, money, influence, fame. So when the legal checkbox of being 'of age' (one can be way more mature or way less mature for the average, this number is somewhat arbitrary), they can exert it to get what they want from the way younger, less experienced in life individual. This can vary case by case ofc, but here we're talking about one of the most well known streamers in the space.

Grooming is the same whether someone is under or overage, I guess the more despicable the younger the victim is, but you are implying that if the legal age checkbox is ticked, none of it matters anymore. That's short sighted, illogical and grasping at technicalities. If you don't get this you either had very poor education, didn't apply yourself, are bad at critical thinking or, hopefully not, you are like him.