In real life, you sit in a room and someone asks "Ok, we've talked about all alternatives, X seems to be the best solution, is there someone against it?" Now you have about 2 seconds to intervene and expose yourself, otherwise consensus will be assumed. It may not always be so dramatic, but in real life you can be sure that you know the reaction of everyone involved.
In an online community you don't know who is "in the room", you don't know whether people will read a question or suggestion, how many people will read it, when they will read it, how much time they think to have for reflecting on the issue, whether they will bother to comment - especially when perhaps others have already commented according to their opinion. So online consensus means total uncertainty. When you assume consensus, at a later time anyone can pop up with "I did not agree" and there seems to be no argument against that.
Both build an some expectations, which may be negotiated, of course. The first one is that people who care will read the wiki regularly and therefore one may assume that they will read the suggestion within 2 days or 3 days at most. (Of course this means that the suggestion mustn't be hidden in a place that draws little interest. For example, it must not be on a homepage or an unrelated page. The second expectation is that there is no silent disagreement - someone who disagrees should say so. Note the asymmetry: agreement can be explicit or silent, disagreement only explicit.