Here's evolutionary psychologist Steven Pinker attempting to explain the same phenomenon of an enlarged cranium without mentioning symbiosis:
Pinker does invoke technology, language, and "division into cultures" - but these are just assumed, and there's no mention of how they fit into a biological or evolutionary framework.
Symbiosis had a hard time being accepted by biologists. Cultural symbiosis is similarly facing a struggle for recognition that mirrors the way it was widely ignored by biologists until the 1960s.
In the 1960s, it was mostly raw evidence (the evidence that mitochondria contained DNA) that finally got the theory of symbiosis accepted into the mainstream, and killed off the dogma that Fisherian bean-bag genetics could account for living systems - which had dominated evolutionary theory up to that point.
The analogous development in the realm of cultural evolution would consist of the discovery for the neuromeme.
However... this isn't the 1960s. We already know about symbiosis. We have an enormous quantity of data, and huge machines with which to analyse it. There is no good reason why symbiosis in the cultural realm should be a neglected idea.
There is really no good reason why the study of cultural evolution should continue to lag so far behind the study of organic evolution. People should just not still be proposing theories of cultural evolution that don't include basic phenomena - such as symbiosis and recombination.
The reasons why such phenomena are currently neglected boil down to ignorance and stupidity. Scientists in the field who are unaware of these phenomena should just wake up.