When Facebook, Instagram, Twitter and other platforms started to offer data exports (even more so on pressure from GDPR) I became excited that there will be great efforts to build upon them, do things with the data that these networks were not incentivised to do. See a timeline where all social media activity is visible at once, see patterns, persons in your life throughout the years, or even migrate to new platforms by importing all your existing messages, content. The data is wast, and easily usable.

Apart from experimental small scripts I couldn't find any projects like the above. Do some legal terms explicitly forbid to use them in this way? Is there really such a disinterest from the dev community to work with this data?

To note: yes, handling such data is a huge privacy risk, but a) these could be offline/self-hosted tools b) if someone trusted the same data to these platforms, why couldn't he entrust them with other more open/transparent/appealing platforms.