Please explain why symlinking doesn't work on updating packages?

[S-Nakib]

Let I have a package-a(version 1.0.0) and another package-b(version 1.0.0). package-a depends on package-b by ^1.0.0.
Now if we run npm install on the root folder. Everything works fine. The node_modules of the root contain both package-a(version 1.0.0) and package-b(version 1.0.0).
But now if we update package b to version 2.0.0, package-a breaks. Because it doesn't find its dependency(package-a@^1.0.0).
The intended behavior would be package-a containing the version 1.0.0 of package-b in its node_modules folder. But it doesn't happen. How to make it happen?