It's ridiculous to have to maintain the two and try to keep them in synch.
The two have the same intent from this users perspective, they put contacts for people into groups.
Why aren't they shared?
This seems to come up periodically over the last 18 months but nothing has happened.
My only guess is that it's not valuable enough to users from Google's perspective.
The value is that when a small business owner has 2000+ contacts (personal and business)
it's a crazy amount of annoying work to have to try to keep them in sync. It's something that
a computer program should be good at.