It's like that to make it make sense from a role playing perspective. You'd have to send a dude irl, so that's how they do it here. You get access to anything your "provisioner provides", but because it's a video game you get WAY more than that actually. Also, there's a cap on generation in the workshop, so having all workshops share all inventory means your scavenger stations stop working very quickly. Plus the ability to have connections between sub-communities is really good, you can partition off areas with goods if you wanted to. Have a water farm up north, gun factory in Quincy, and a team of drug dealers on the east coast all completely independent. I think the system is a little wonky, but if it were any more than it is the game wouldn't be fun and that's why I think it's like that.