They didn't all agree, though. Now there's Ethereum (the people who agreed with the fork) and Ethereum Classic (the people who didn't agree).
Your first comment [1]
> As long as everyone agrees i see absolutely no problem
and now
> Some agreed, others didn't...Consensus doesn't require everyone to agree, just the majority
So you literally just said there is a problem.
The DAO was a very extraordinary event that occurred under very unique and unrepeatable circumstances.
1. Ethereum had just launched. There was a sense of everything being in beta.
2. The Ethereum stakeholder set was very small, so obtaining consensus for such a controversial hard fork was much easier to accomplish than it would be today.
3. The economy on Ethereum was very small, so
a) a hard fork was far less disruptive than it would be today.
b) an undermining of Ethereum's commitment to neutrality/immutability jeapardized far fewer decentralized application projects than it would today.
4. Smart contracts were completely new, so there was a sense that people could be forgiven for their mistakes.
5. The Ethereum Foundation had promoted the idea of a DAO on their website, and several Ethereum founders had promoted the specific DAO that ended up being hacked. These facts made the DAO appear to be more than a completely third party app.
Ethereum is very different today than it was in 2016, and a DAO-scale mishap would never lead to a hard fork again.