Why would a PCIe x16 slot be limited to PCIe x1 card?

Depends on the case and where your PSU is if putting the GPU in the bottom PCIE slot it might reduce airflow to the GPU with it being to close

The PEX4TO16R PCI Express x4 to x16 Slot Extension Adapter lets you install PCI Express cards into x4 PCIe slots in a 1U rack mount chassis

can you put a pcie x4 in a x16 slot Yes PCIe Express connectors are designed so that a larger slot can accommodate a smaller card So, a x4 slot can handle x4, x2, and x1 cards

candy rush slot demo Some do On some of the motherboards with PCIe 16x slots hardware configured to the full standard, it does