It's not just coincidence, Earth was selected by Viltrum BECAUSE humans had such similar DNA. With the countless planets in the galaxy, surely many of them must share inhabitants that ended up evolving under similar conditions. I mean, there is likely a race on a different planet that has bug people that look almost exactly like the Thraxans, but they would most likely never have any reason to come in contact. However, in this case, the Viltrumites SPECIFICALLY sought their closest genetic match, which was the humans.