Are they though? GSP/Silva, Jones/Cain, Jones/Ngannou, Pettis/Aldo back when they were both champs, Brock/Fedor, Fedor to the UFC, Rousey/Cyborg and so on...You could even argue Mayweather v Pac was one of them, I mean they did fight but both were past their best.
I love both boxing and MMA but at least these big fights are much more likely to happen in MMA.
Boxing does get most of them done (probably at the same rate UFC does) but some people are just cowards (Bud/Spence). Usyk, Canelo, Inuoe are all unified champs. Those fights are happening, we saw Fury/Wilder. It's only a couple we seem to fixate on (rightly so as they should be made!).
Yet another reason to dislike Wilder too. Absolute w***er