New tech in the series?

I love this series but this question has been really bothering me.

Spoilers ahead maybe? I dunno.

Why is no new technology developed to fight the cielcen. They fight them for over a thousand years by the time it’s all said and done and nothing changes. The humans use the same defenses and weapons the whole time whereas the cielcen use the extras to add other weapons to their arsenal. Anyone have any thoughts?