4.) Gameplay isn't as polished
Not all old games have bad gameplay, but generally it's not just because of the developer's fault. The tecnology's limitations at the time also play a role in what can be done. Perhaps, the A.I can't be programmed to do things that our enemies can do now.
I'm sure that it took years of trying and observing to master the perfect inventory system or combat. Great games don't just come out of thin air. They are a product of improvements and inspired by other games that came before it.
