Returning an array would be much more expensive than returning a tuple, as an array has a significant amount of overhead compared to a tuple (which can be exactly zero-overhead in many circumstances), which is why it’s not done. Also, I’m not convinced that that would help in your case anyway, as a vector-of-vectors is still a completely different structure than a 2D matrix, and something like A[:][1] would not work even if each element of A were a vector.
Maybe you can describe more about what the actual problem you’re trying to solve is?