Thanks for the question. Optimization is a great but tricky topic. I would guess that the software that you're using has a parameter somewhere tucked away that is forcing it to only give these solutions. As I'm sure you know, it's very common in optimization for the optimal solutions to be corner points. If this is a situation where you're looking at the result and you intuitively know that it should be offering a different solutions, then maybe the model needs an additional constraint and/or additional consideration in the objective function (i.e. is there something in your head that's not yet in the model?).
In any case, good for you for critically questioning the optimization results! I hope this helps.