>>1505890>>1506067>>1506088Admittedly not a great defense, but there was simply no more content in the light novels, in fact the only times that were covered in the original set of SAO arc books are the 1st episode and the 2 year time skip. The rest is taken from the side stories.
My understanding is that it was only supposed to be an adaptation of the books so making new content wasn't really on the agenda.
