If I recall correctly, at one point, the network interface was used as a source of entropy. Then someone demonstrated that sending the right sequence of network packets to a machine would let you control the key that got generated. So they removed it.

Then folks discovered -- in production -- that some cloud computing environments just don't get any other new entropy after boot, and so instances would hang on generating SSH host keys.

Some folks went to /dev/urandom. Other folks decided to seed instances with entropy from another computer (with fancy names like "cloud entropy service"). And then someone had to decide how that machine gets entropy (like plugging in an FM radio into the mic jack).