How are PCIe Expansion cards not way more popular

Onboard m2 slot or pcie m2 expantion card? which is faster? m2 expansion slot a single slot x16 PCI-e expansion card for 4x NVMe drives: productsasus-hyper-m2-card-pcie-30-x16-4x-m2-pcie-2242-60 m2 nvme slot The number of links, or lanes, is denoted on PCIe expansion slots and PCIe slots by affixing an “x” followed by the number of links to the

m2 slot mainboard -Is it possible to split an slot into 2 or 4 slots? Some motherboards do support PCIe bifurcation, but even on systems that do support