It’s funny how much we limit our understanding of the past by our own current limits.

Back in the days I had quite old teachers who knew few dozens long poems by heart and most of them learnt in their youth.

I think that the way we manage our memory and the context as well as the ratio short/long term memory demands scoped to that context is totally different and is diverging from old times.

As for the writing, there are many instances of long novels where the author radically changes the style even nowadays.

Last but not least, we have inherited a disfigured version of those epics. The originals sung by their authors are long gone. Thus the iterative divergence by the following generations of bards in terms of style and even in addition/deletion is inevitable.