Since one of the biggest advantages of our species is our brains, it makes sense that evolution would produce slightly different types that give different advantages. Given that we're a social species, having a variety of neurotypes in a group would offer all the advantages while the group as a whole offsets the disadvantages like inattention.
Imagine a group of hunters. Someone with ADHD might hyperfocus and be really good at spotting fast prey like rabbits or birds, but may not notice a wolf creeping up on them. The group can compensate for that, and are more successful than without the ADHD tracker.
Just think about the wide variety in our bodies. There's a whole spectrum of physical ability from very small and nimble humans to very big and very strong ones. There's a huge range of different adaptations evolved by different groups. It really just makes sense that we'd see a similar sort of variety in structure and ability of something as complex as a brain.
Humans evolved to fill niches, same as any animal.