Then why did most software fail to do that even before the advent of LLMs?

Because designing systems that work well is difficult. It takes years of experience to develop the muscle memory behind quality systems architecture. Writing the code is an implementation detail (albeit a large one).

Are we sure it's not failing anymore after the advent of LLMs?

Because coding bootcamps and CS programs were churning out squillions of people who could type the code but had poor design and analytical skills, because there was a time where being able to implement Dijkstra on a whiteboard would get you 400k at a FAANG.