Most consumers only have 1-2 NVMe drive and a GPU. So I presume OEMs don't want to have an M.2 slot buried under the GPU. On AM4 ATX-boards slot 0 is often the

Most consumers only have 1-2 NVMe drive and a GPU. So I presume OEMs don't want to have an M.2 slot buried under the GPU. On AM4 ATX-boards slot 0 is often then primary M.2 slot. Slot 1 the GPU and slot 2 is either empty, or the CMOS battery, or like you suggested a secondary M.2. So you're already down to a maximum of 5 PCIe slots.