An opinion would be "guessing" about why they picked a certain opponent if they offered no explanantion. The facts are that they put their bodies through a grueling and dangerous process to "fit" into smaller categories and that makes the "WHY" a fact. "Why do they do this?"...to avoid bigger and potentially better competition. Khabib doesn't fall shaking to the ground the day before fights because it is good for his health...he doesn't almost get hospitalized BEFORE he even fights because he is looking to prove he is the best fighter...he does it because Usman, Covington, Thompson, Romero, GSP, Bisping, Santos, jones, Blachowicz, Adesanya and a host of other, bigger and tougher opponents would be awaiting him. How do you think he would do against these guys? Apparently, he doesn't think he would do so well because he nearly kills himself before every fight to avoid them.