One of many reasons for this could be your choice of topics, the location of the argument, and your perception. Any Centrists in the argument may be on your side of that particular issue and therefore are appearing left-leaning to you at the moment.
You not looking for them does not mean that they are not there. Them not coming out and identifying themselves continually does not mean they are not there.
Just like anything in science, your perception is not reality, especially when it seems that you are looking to enforce an opinion, not find the truth.
Most of the Centrists I’ve spoken to also tend to argue the issue, and not the side. They see sides and at-all-costs group membership as a form of lunacy.