Because it's a gravy train. Reports MMA Weekly, 4/8/18:
Brooklyn fans showed their appreciation for the fighters to the tune of 17,026 in attendance – a sellout – accounting for a venue record gate of more than $3 million, according to UFC [Ultimate Fighting Championship] president Dana White.I think he meant highest-grossing sports event.
“This is the highest grossing event in Barclays history,” White said at the UFC 223 post-fight press conference. “Madison Square Garden, we’re one and three. Here, we’re one and two.”
Updated: Indeed, MMA Weekly got the quote wrong. USA Today, for example, used the term "sporting event."
For example, as noted by the Barbra Streisand archives site, in 2016:
Barclays Center, the only two-show engagement, scored $8.8 million in sales from 29,250 sold seats on Aug. 11 and 13.