First, most data is not useful without the application that created it. One of the most ironic issues with Google Takeout is you can download data for all of your Google services but import it into almost nothing, there's a handful of nerd projects that can import some of it but most of it requires you be a programmer yourself to use. So the user really should own their data and the app needed to use it.

Second, as soon as a company-hosted app can access your data it can copy it. So keeping your data separate in a pod isn't meaningfully helpful for privacy either. If you are connecting to their servers to use an app, it can be proprietary and you can't see what it does with your data, so you can assume it isn't private anymore.

There are a lot of better implementations than Solid. But since Tim invented the Web, his implementation continues to get press on major news outlets even though it's a bad idea that's gotten no traction in over a decade.