It's not a contradiction, but whatever. The constitution is not a really good document in general, I'll agree with you that the 10th Amendment is generally interpreted pretty broadly.

However, as it stands it doesn't have this glaring flaw that you imagine it to have. It just doesn't make sense to have a loose confederation of states that govern themselves. It made sense before we could communicate efficiently, because we needed local governments to enact country-wide legislation, but not now. We're not these isolated pockets of communities or cultures anymore.