Do they?
2018 champions defending their titles:
Whitaker - 1
Woodley - 1
Ferguson - 0 as champ .. had to relinquish
Khabib - 1 as champ, had another fight as contender for the title.
Holloway - 1
Dillashaw - 1
DJ - 1
Cejudo - 0 as champ
Namajunas - 1
Nunes - 1 defense, 1 going for the title above her weight which it seems she will never defend.
Stipe defended 2 times
Cyborg defended 2 times
Cormier was the busiest champ overall with defending both belts 1 time each and winning the HW title for 3 full fights. I'lll cut him slack for not defending either belt twice since he defended both for 2 title defenses in total even if its different titles.
Bonus - Jon Jones has averaged a perfect 1 fight a year for the last 5 years.
So where is this majority of champions fighting 2 times a year? Only 3 put their titles on the line more than once last year.