So what.
Ye, because Cro Cop actually fought during his career, which fighter should do, and gave opportunities to all comers to fight him; while they did not. They fought once per year, calculating, and avoiding taking risk. Modern MMA makes a taboo out of losing (guess UFC drew it's analogy from professional boxing) and taking fights and aditional risk. If you did not fight additional opponents, that does not mean that you are unbeatable. If you fought only a few top opponents, it does not mean that someone lesser known HW can not beat you if he has his day. You have an example of JDS who is basically the only modern fighter HW who fought from his early 20s to late 30s, and as soon as he accumulated a little bit more matches, he came from unbeatable look, and 15-1, to 10 losses. Fighter should fight as many different dangerous opponents of different profile as possible, and not bussiness optimising his career taking as few matches as possible. And they got their shots to get titles and belts when it was their turn, whie due to unforseen circumstances when it was Mirko's turn to fight for no. 1 place against Fedor, he instead fought anonymous Gonzaga in rules that totally changed over night.
The more you fight, the more you will lose; the more different MMA rulesets you change, the more you will lose.
The claim that fighter should fight as less as possible in order to avoid losses, so he would "look unbeatable", which is a modern paradigma, is apsurd and against fighting spirit. Credit goes to fighter who fought more, got more defeats, took more risk, but significantly more good wins, than to fighter who fought less in order to fakely look "allways" good and "unbeatable".