Those ones who use multiple alt accounts usually don't care. They change campaigns they participate easily, they don't care if one of their accounts is excluded from participating, they will make several new. They are not interested in what they are talking about. They don't use notifiers because they don't care if they are answered or mentioned anywhere. I'd say that OP is not the only one who is a bit annoyed when there are users who post a lot but don't ever take part in discussions. 
I think another reason why they do not participate in discussions is that they usually do not read previous posts and do not delve into the essence of the topic, so their posts fall out of the general narrative, and few people want to write comments on their posts or mention them. This is really annoying and confusing. And this is also another problem associated with alt accounts.