Security at its core comes down to trusting the supply chain that provides the software that runs on your hardware. There are many alternative secure supply chain models, but ultimately users often are incapable of making great choices on what is trustworthy. It does generally make sense that the OS vendor needs to play a key part in helping ensure trusted parties are involved in the supply chain.

Comparing phones to PCs isn't a great comparison because PCs don't have a great track record and the amount of personal data and ease of installing lots of apps is quite different. Of course the current arrangement is far from perfect, but acknowledging the problems it's trying to solve is an important step towards trying to find a solution that is better.