Because when you open and later refresh Lemmy, the quantity of new posts you will see, which gives an idea of Lemmy's activity, is proportional to the absolute number of active users, not the proportion of active users per instance.
By why not both, I meant why not display both the number of users and the proportion of the total userbase. This does not seem to be an answer to that question.
I thought you asked why wouldn't both be felt by users. Otherwise, sure both can be displayed, the proportion gives an idea of the dynamic of a community.