The AMD socket AM4 provides for up to 24 PCI Express lanes. The CPU has 32 lanes but they are not all presented to the socket. Skylane is even worse with a measly 20 lanes.

The limited number of lanes is of concern mostly as we prefer to have more slots with at least x1 support for selected peripherals.

IEEE 1394a is available for PCI Express and this allows legacy video hardware to continue to be used. SATA cards are also easy on lane requirements.

WiFi with Bluetooth needs a slot and a USB header connection to function, which is lower cost than adding a USB chip to the logic board.

M.2 slots need 4 lanes which is where the demand is growing at present. ATX motherboards now often have dual M.2 slots which allow for increased SSD storage.

So the video card can take 8 lanes, the M.2 has 4, there are 4 for the southbridge chip and this leaves 8 lanes for the rest of the motherboard. For this reason it may have been more prudent to have the full 32 lanes available so that devices are better served. The desire for more 4-lane slots is the main idea. The uses for a 4-lane slots are abundant.