Not to speak for anyone else, but I don't think that was borbole's point
phpBB is the most popular forum software in the world - this is not disputed. However, borbole was commenting that MyBB is actively used by quite a few forums, which is a different point altogether. I wouldn't dismiss MyBB simply because phpBB exists, because it has its own merits and people do use it.
More to the point, this thread is asking why there is no MyBB importer, so presumably anyone reading would already be on MyBB - so whether phpBB is superior or not isn't really relevant
I've already shared my thoughts on the matter: XenForo has importers for phpBB, vB, and IPB, which covers the most popular forum packages. Anything else can likely do an interim import to one of these three. It would be nice to get a MyBB importer, but (in my personal and utterly unofficial opinion) I'd rather see resources being focussed on XenForo itself.