It is easy to grasp why modern IE languages are simpler than ancient IE languages and why they show signs of ongoing simplification but why were the ancient languages so complex to begin with?

Ancient Indo-European languagues only seem complex compared to modern languages. The reason is evolution. When language first began, it was problematic and incomplete. No one set out to create a complex language. The early language users were just inexperienced.

Over the millennia, problems were solved and shortcomings were weeded out.