I think there was another thread about this already, and somebody there explained really well why so many show so many humanoid features. In short that's for story telling, immersion, interaction, relatability (if that's even a word) and most of all empathy.
I mean they threw enough other stuff in there like rachni or elcor which were not so conventional. As for the fancy pants version of Aliens, we have no way of knowing how and in what form life could exist outside of the existing carbon based form. Therefore it's difficult to design something for that purpose.
Which brings us to the next thing, intelligent life without carbon seems kind of unlikely with known elements. The reason carbon is used on earth as basic life building block is simple. Carbon can bundle and connect with so many other elements. We have no other element which can do quite like carbon and that is very important for life. Anyway ME is already a bit on the fantasy side of things and adding some super trippy aliens wouldn't really help that (yes I know arguably that is more realistic but it's more about the image it projects rather than it really is)
But what do I know, I'm just a penguin and now...
Peter!

Well, the chemistry of silicon can form elaborate branching structures in a molecular tetrahedron (with single bonds) as carbon can, and can be replaced for carbon in almost every simple carbon backbone with largely the same chemistry.
However, the energy required to break even single bonds between two silicon atoms, a silicon and a hydrogen, etc is comparatively small compared to that required for carbon. This makes it unlikely that silicon based life forms would be commonplace, as it would be unlikely that the same degree of elaborate biochemistry could be possible with silicon in most situations.
Keyword most. In this vast universe of ours, there are probably very specific situations where silicon life could be possible and thrive. But carbon is almost ubiquitous, and carbon based life would almost certainly outcompete silicon based life wherever the two coincide.
So I wouldn't be upset if Bioware explored the possibility of silicon life, provided that they pay attention to the real world science of the chemistry of silicon and the reasonable speculation of how it could or couldn't work with biochemistry.