I can't speak to people's intrinsic enjoyment. That seems very personal.
But I do imagine that there are different skills, techniques, tips, ideas, etc. that one can gain by trying out the different modes. Things that one discovers while playing career that makes one better at sandbox, or things that one learns in the franchise community challenges that help make you faster at solving a timed scenario problem, for example. And I think there might be many players (can't speak for every one), for whom playing a different mode might give them new ideas for laying out their franchise zoos, or allow them to explore in-game techniques that end up improving the habitat design in their sandbox zoo in a new way that they might not have thought of before.
Most games have you balance multiple tasks, just like most sports have offense and defense. Frontier has very generously made it so that we can pause or turn on/off a great many of those features in different ways and in different modes and difficulty levels, so that we can kick/throw/hit/catch/run/tackle/ (or build/manage/create/breed/buy/sell) all day long if we want to, without having to think about the rest of the game. But it seems very odd to me to expect that every benefit of playing the full game in all modes absolutely must be achievable when playing only one portion of it alone, regardless of the undeniable amount of time, talent, passion, and creativity put into that one part.
Yes, there are places you can go that have putting greens, and batting cages, and throwing/kicking the ball around. And people gain hours of enjoyment from those, and can get quite good at them. But if Planet Golf had a putting only mode, or Planet Baseball had a batting only mode -- and further allowed you to "disable pitcher welfare" so that they threw strikes every time -- I'm not sure that I could honestly complain if playing that mode alone didn't unlock every other feature of the game/sport. Even if running the bases of the other mode doesn't have any intrinsic value or enjoyment to me personally.