As far as I know, the counts per minute for background radiation you get with cheap compact captors (say the stuff you find on Geiger counters) is pretty low, a dozen to a hundred counts per minute. That's not a lot of entropy. You could much more precise captors, but then you couldn't just plug it into a PCIe slot. At this scale, you probably get more entropy from just jitter on the electrical circuit. And it's pointless to have one huge centralized RNG, at least from a security standpoint.