Hey I get one of these!
Backwards compatibility with X370/X470 probably. The reduced complexity of the IO/chipset dies would make it cheaper to produce as well.Why not use IF to connect the chipset to the CPU? Instead of using pcie Lanes.
The same can be asked of why Navi can not use IF to link to Ryzen 3 when they use it in Vega II Duo (84GB/s bi-directional link on that one btw).