They also never explained the pyramids way before D1 release. Not everything needs to be explained immediately though. The Leviathan sword is called "It stared back". 4 years later that sentence came back again in the Lore books for the Presage mission. And it explained that, whatever stared back, was an actual entity of the Darkness Calus met during his very first encounter. Back then most people just thought it was bogus and Calus' way of talking. Yet, even Katabasis saw it as we later discovered. Or the mention of Calus Daughter in Year 1 + her decision in the much later released Heir Apparent exotic which lore states that she actually wants to hunt her father down. And in Year 4 she finally appeared in the Sol System with her fleet. In the Lore and even during gameplay Bungie has teased us with tons of stuff. Little and big things which later came into play in terms of lore/story development. It's a nice thing to let people speculate and keep the lore and universe busy. And people do enjoy that. Lore channels on youtube or communities are very popular when it comes to Destiny.