The syllogism's premises are not proven, they are assumed. It is really not relevant if Socrates is an alien android. IF the premises are true, the conclusion follows. We go along with it easily, because the premises happen to "seem" true. Tell someone: ALL A is B, C is A, therefore C is B they might not view it is tautological. So I think part of the answer to your question is that verbal / linguistic operations take place more automatically than formal symbolic manipulations in our mind, having been learned at an earlier age. It's also a valid syllogism to say:

All alien androids are mortal, Socrates is an alien android, therefore Socrates is mortal.