Why does cardio vary so much in MMA?

I never hear things like" Kevin Durant is gassed here in the 4th quarter" or guys really gassing badly vs opponents in other sports at the highest level.

I know team sports have substitutions and coaches can monitor and manage players' minutes so there's that but you would imagine every UFC fighter is fully prepared to go the distance of their next fight and that many times isn't the case.

I was watching Holloway x Aldo 2 tonight again and Aldo gassed so hard in the 3rd while Max was...

Why does cardio vary so much in MMA?