pci express - Why can't PCIe devices share lanes?

On older chip-sets it was not possible to have a dedicated graphics card in the PEG slot and have the Integrated Graphics Device (IGD) enabled at the same time.

After looking online I found this was because they "Shared the same bus lanes" so it was either one or the other and the BIOS would disable the IGD if it detected a card in the PEG slot.

I was just wondering why PCIe devices can't share the same lanes. It is my understanding that with the old PCI standard all devices shared the same lanes so why is this not possible with PCIe?

I know PCIe is serial and creates point-to-point connections between the Switch and 'End Devices' but I don't understand why if the IGD and Dedicated card share the same lanes why they can't work together?

What part of the PCIe design prevents this?

Thanks.

I was just wondering why PCIe devices can't share the same lanes.

PCIe uses a point-to-point topology, so each lane expects one device on each end. If it wasn't, it'd be something like the original PCI - one issue with plain PCI is that when one device is talking on the bus, others have to wait.