There's also the fact that Astromech droids aren't really intended for frequent contact with sentient beings. They're mechanics - that's it! Nobody cares about what droids have to say anyway and certain droids are given speech modulators only if it suits their function. Threepio has speech because he obviously needs it as a translator.
The droids was something that Lucas apparently got from Kurosawa Akira - the idea of telling a saga through the eyes of the lowliest members of society. In Kurosawa's 隠し砦の三悪人 ("The Hidden Fortress") we see the tale through the eyes of two comical peasants, the lowliest class in feudal Japan (especially compared to samurai warriors and princesses etc.). In the Star Wars universe droids have the lowest station in society - arguably even lower than slaves because they're not even living and absolutely nobody cares about their welfare or opinion whatsoever!
"Let the Wookiee win."
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