The source code needs to be on everyone’s computer so that its execution is decentralized.
Part of the problem is confusion around the word decentralized. If I have a website billprin.com , in a sense it’s decentralized because there’s a million websites and that one is my little corner and you have your corner.
But in another sense it’s centeralized since I get to be the sole arbiter of truth on billprin.com
On ethereum, everyone runs the same code and has the same state so nobody is the sole source of truth.
Blockchain is way slower and more expensive that a centralized db but it has this key feature that no single party controls it. This enables us to write applications like automated market makers that require no trust in a centralized party or more choice in whom to trust and more granular control over trust.
It also has the nice side effect of making a lot of stuff more transparent and programmable.
When do we prefer not to have to trust a party? Many people don’t trust Wall St entities so are excited about potential ways to decentralize finance.
Others don’t trust big tech companies and are excited about ways to decentralize identity and communication.
And the future is hard to predict , it feels plausible that a big blank canvas will lead to more applications that are hard to predict.
The key is realizing that the core feature is not having to trust a centralized entity.
There’s technological challenges , and ways in which the current landscape doesn’t live up to the ideology. The money both incentives early adoption and development and turns it into a casino. But hopefully that helps clarify why some people are excited about the underlying potential.