Typical consumers have no way of ensuring their UI is, in fact, encrypting the data and not farming it out. They cannot verify the source code themselves, because they don't have the technical skill set they'd need to do so (nor, frankly, the time). They're reliant on the goodwill of whoever packaged and installed the offering for them not doing anything to that offering.
Technical power users can circumvent this because they can build/install from source, verify keychains, read the source, etc. Non-technical users can't do this, and need someone to help them. That someone will most likely be in the form of a third party organization that does this in exchange for money. They're placing their trust in that third party.
The point I'm getting at is that, eventually, a consumer has to trust a third party who may have incentives that don't align with their own. They're just playing a game of which vendor to place that trust in. This is why centralization is still the predominant architecture choice for the overwhelming majority of products, even in a world where myriad decentralized solutions exist for almost everything. It turns out that having bespoke third parties run decentralized solutions for customers is often not a better product experience, and still has the same root problem even if it manifests in different ways.
