Boxing is far more dynamic since it isn't turn based and exploiting tactical opportunities relies on your speed. Not to mention actual the actual strength required to make said exploitation meaningful.
As for chess Carlsen knows he's the best of all time, why should we hold it against him if he himself knows it? It's like people getting mad at Kasparov for admitting that he's better than Fischer.
I find alot of people being anoyed by some of Carlsens repliques. That said, compare Carlsen with any boxer. A lot of confidense combined with arrogance, but in the end, after the match the story is somewhat different. "He did well" or "I did poor". Same as in boxing. Just saying.