@aodoherty I'm not sure why but I also rarely see any of my max. Fitness (level 10) Sims spawn at a gym. Whenever I've checked NPCs at the gym (by using MCCC) it seems like their Fitness level is usually between 4 and 7. I know I've been able to prevent certain Sims, like Eliza Pancakes, Darrel Charm, Paka'a Uha, etc., from spawning at gyms if I cheat their Fitness skill back down to 1. I've also been able to make other NPCs appear regularly at gyms by cheating their Fitness skill to 4. It doesn't seem to matter whether or not they have the Active trait, I can basically control which NPCs spawn at gyms by cheating Fitness skills up or down. This is how it seems to work in my game anyway & I am basing this just from casual observation rather than any actual analysis of the game code. Even so, whenever I send my Sims to a gym, I generally just see the NPCs I want to see and none of them that I don't want to see.
I know this doesn't really answer the original question about why Sims with max. Fitness skills don't seem to spawn at gyms, except to say that this also appears to be happening in my game.
I'll ask around a bit & see if I can learn any more about this.
EDIT: Ok, I did some asking and it sounds like it might be a tuning issue where the game is not set up to spawn Sims after they have reached the maximum Fitness skill level. It doesn't sound like it's fixable outside of using mods.