The graphics cards have to interface through PCIE. The Pi simply doesn't support that interface.
Something like a LeMaker HiKey 960 has PCIE gen 2 support and an m.2 interface... so maybe with an m.2 to PCIE slot converter and the software to make it communicate with OS you could run graphics cards. It costs $250 for this device though, so not really saving money compared to the typical x86 motherboard and cpu combos we buy today.