Modern retro computer designs run into the problem of generating a video signal. Ideally you'd have a tile and sprite based rendering. And you'd like to support HDMI or at least VGA. But there are no modern parts that offer this and building the functionality out of discrete components is impractical and unwieldy.
A FPGA is really just the right tool for solving the video problem. Or some projects do it with a micro-controller. But it's sort of too bad as it kind of undercuts the spirit of the whole design. If you video processor is orders of magnitude more powerful than the rest of the computer, then one starts to ask why not just implement the entire computer inside the video processor?