They get those PPV numbers because UFC is oriented towards the PPV market and has a huge fanbase that will buy no matter what simply because it's UFC.
The UFC has what, around 8 stacked UFC PPVs a year where almost all the relevant fight in the entire MMA take place. If you'd stack all the relevant fights in boxing on 8 huge cards then those cards would generate crazy numbers as well.
For example if you'd put all the relevant boxing fights in September on one big UFC-style card that's GGG vs Brook, Canelo vs Smith, Usyk vs Glowacki, Crolla vs Linares, Chocolatito vs Cuadras and that Naoya Inoue fight on one single card that would generate huge interest globally (because a lot of those fighters are big names in their own countries) not just on American PPV market. Then for October-November PPV you'd have Ward vs Kovalev, Pacquiao vs Vargas, Nonito, Parker, Braehmer vs Cleverly, Tony Bellew, Quigg and Anthony Joshua on the same card.
UFC "stars" benefit from the UFC format. The only active fighter who'd sell big on his own is Conor.