Those MBs are cheaper precisely because supporting this kind of bandwidth breakout adds cost (-> a fancier PCIe switch in a higher end chipset/southbridge). If you add the support to do this to them you end up with the more expensive motherboard. Some of the highest end motherboards actually have 2 chipsets/PCIe switches, more cost but a more bandwidth sharing.

You can also buy external PCIe switches (just make sure you're not accidentally buying a PCIe bifurcation device). Most of the time it's cheaper to just buy the higher end motherboard though, e.g. I don't want to know what price "Request a quote" for this PCIe switch which can do x8 4.0 upstream and then quad 4x 3.0 downstream https://www.amfeltec.com/pci-express-gen-4-carrier-board-for... is. I do have a few 3.0 era cards which were more reasonably priced though https://www.aliexpress.us/item/3256801702762036.html?gateway... and they've worked well for me.