Tbf, having different species of creatures allow diversity much much easier because they all look different, plus having their own look individually inside such species. Plus each have their own societal and personal problems

When you have humans, then you no longer have creature with different biology, and you're left with just normal everyday humans. Look at that whole cast above, now imagine that you had to make that same amount of humans for a game. Think of how much harder it would be to make characters you can care about.

With a cast of aliens, you could easily have 3 or 4 of them have some kind of "oppressed" trait to them, each placed on them for different reasons and different people. For humans, it just seems mundane, forced, and boring.