Because the establishments had become corrupt. They didn't start out that way as they were established by God and nothing God founds is imperfect. Not only that but you aren't accounting for the Holy Spirit, who gives men the ability to attain true righteousness. Just as the spirit dwelt within the apostles in a special way of establishing dogma through Christ's teachings, and they were especially authoritative in that way, so we're the men they appointed as elders. Why even have elders if they aren't there for guidance and leadership. That verse is addressed to the Church and is surrounded by verses aimed at the church.
If we had chosen the Most High God as our King, we wouldn't have to worry about any corruption of institution. That is part of the point - we never learn. We have a perfectly Good God who wants to be a Father and King to us, and we keep asking for human substitutes for the real thing.
The Word of God Himself said that where two or more are gathered in His name, He is in the midst of them. That means only two people are needed to have Church. Indeed, the Church is beyond an institution;
it is a people.
Why have elders indeed? They are there for guidance and leadership, not establishing ecumenical systems of control over laity - deciding which
scripture, for example, is good for enrichment for the rest of our souls. We are responsible for the trajectory of our souls. The verse is an allusion to when Israel was a
people, when the
elders and prophets were not intrusive/imposing and demanding audience for their teaching, and when those leaders
followed the Word of the Most High God first, instead of following a system of verification and vindication set up by men.
Everyone has everything to offer spiritually; while leadership and teaching is a distinct quality, there is no reason to make an institution that exalts other humans in a position of power. We see
throughout history what that breeds, and we keep running after it! It is lunacy. Our Father gave us a very easy way to have a relationship with Him and we have substituted almost every way for some
image we think is more appropriate.
Even a debased leadership all around the world wont get people to stop trusting in systems, and just follow the Most High God? I am not going to argue against someone who wants to be swindled. It isn't "my agenda", "my narrative", or "an incessant sense of pessimism about the Church and its members". This is just
history; why do we keep fighting to repeat history - especially when its results are something we see as we live?