Why are so many Star Wars species so similar in external anatomy?

Question

There are so many different species in the Star Wars universe yet many of the sentient ones have the same general humanoid appearance: Two arms, two legs on a torso with a head on top. I'm not saying there aren't intelligent species that vary from this, I'm just wondering why there aren't more?

I thought it might be the most "naturally selected" evolutionary development, but that seems unlikely since planets can be so vastly different. There are some that are almost completely water, others that don't have oxygen and still others that all forests. Why would they all develop the same shape of intelligent life? For example Manaan (Legends Canon) is all one big sea with the exception of a few space ports for foreigners, yet the Selkath are bipedal and not entirely fish like.

Answer 1

Canon:

I couldn't find any in-universe reference. Convergent evolution is a convinient hypothesis, but has no canon evidence.

EU/Legends:

Many, though not all, of those "near-human" species seems to be genetically descended from humans, according to what pretty much passes for an expert (Caminoans):

"Above all else, humans are adaptable — adaptable in terms of physiology, mentality and society. Their genome is remarkably elastic: Selection pressures need a few millennia at most to engage new genes and reshape their bodies in response to environmental changes, as the galaxy’s countless near-human populations attest."
("Star Wars: The Essential Guide to Warfare Author's Cut" - published on starwars.com; so there's a chance it may be Disney canon)