Availability heuristic. We are the only example of "human level or greater" intelligent life we know of, so we assign "human like" needs and wants to these hypothesized aliens because it's all we have. I think the key thing is just to remember that this is just a heuristic and not to be overly rigid about it. Your point, IMO, absolutely stands that there is no particular reason to assume that "aliens" must be similar to us in this regard.