The issue is not "access" to your data. You're not forbidden from accessing your data in any case. The real issue is that it may be bad to have full and sole responsibility for maintaining backups of your social media data.

1) If you add image files and especially video files to the mix, the size of your data can get huge.

2) Smartphone users especially don't have a ton of free storage space.

3) In general, people aren't great at maintaining their own backups.

The question is why a so-called distributed network can't maintain distributed copies of user data, rather than forcing the burden onto the users.