We constantly want great fighters to go up a weightclass to prove their greatness.
Actually fighters going up in weight is a boxing tradition. In grappling it almost never happens. Satiev for instance, often called the greatest freestyle wrestler ever, won 3 Olympic gold medals and 6 world championship gold medals, all at the same weight. No one ever said he should go up in weight for the next Olympics to "prove his greatness."
Same for Masae Ueno, the woman who won gold in Rousey's weight division in judo (she won it twice, and two world championship gold - she was better than Rousey on the ground and on her feet). She's famous as a dominant champ, and no one ever suggested she go up in weight for the next Olympics for a challenge.
And you can go through a long list of wrestlers and judoka, who stay in the same weight class from Olympics to Olympics, never going up in weight, and who are revered in wrestling and judo.
So ask yourself, why should Rousey follow the boxing tradition of going up in weight, instead of the grappling tradition of staying in your weight, given that her background is grappling? Especially since pure grappling beats pure striking more times than not - wouldn't it make sense to follow the tradition of the more effective background sport?
And I note that if going up in weight is the point and a lack of organized weight divisions isn't an issue (ie it doesn't matter that the UFC has no 145 pound division), then shouldn't Cyborg be arranging fights at 155 to challenge herself, instead of wanting to fight a smaller fighter?