Maybe you should watch the movie "Animal Farm"?
Complex societies collapse for the same reason smaller systems collapse: corruption.
Instead of something huge like an entire nation or culture, let's think about something smaller. What stages of development does it go through? Perhaps a small club or guild. Let's even look at an example where it starts under the best conditions possible and given a great set of honest, hardworking, trustworthy leadership to start.
1. At first nothing exists.
2. Then someone with a genuine passion for an idea plus a willingness to work hard creates a small club.
3. They recruit like-minded people and it grows and succeeds.
4. With this growth might come some wealth and power.
5. Then others come along who are greedy and corrupt and just want to take power and wealth without working for it.
5a. Some may try to say the wealth and power is not distributed fairly and thus discredit the original creators.
5b. Some may use personal attacks to try to paint the leaders as evil.
6. They stir up anger within the ranks and get the original creators overthrown.
7. This new leadership now takes control of the reins merely to promote themselves and take advantage of everyone else.
8. The club changes to a "lower level" of intelligence where its all about the leader discrediting their political opponents to maintain power instead of trying to achieve any goals, usually via personal attacks or cries of unfairness.
9. The passion drains out of the club, it may limp along for a time.
10. Finally it collapses under its own greed and hate.
Great societies are born from honest, hardworking, trustworthy leadership pursuing ideas. They die when those leaders get replaced by other leaders who use personal attacks and cry about unfairness to maintain power and control.