Most services now offer a data export. Receiving the file and being able to use it elsewhere are different things, and the gap is where portability usually fails.
An archive is not a format
Exports often arrive as a folder of files in whatever internal shape the service uses, which is complete but not designed for anyone else to interpret.
Common formats such as spreadsheets or structured text describe how the file is encoded, not what the fields mean or how records relate to each other.
Two services can both export the same nominal format and produce structures that share almost nothing, because the meaning was never standardised.
Relationships are the hard part
Individual records usually transfer cleanly. What breaks is the connective structure: which item belongs to which collection, what refers to what, and what order things were in.
Those relationships are frequently expressed through internal identifiers that mean nothing outside the originating system.
An importing service must therefore guess at the structure, and it typically reconstructs a simplified version that loses the organisation the user built.
Some data cannot be moved at all
Derived information such as recommendations, learned preferences and ranking history is usually treated as the service's own rather than the user's.
Content that involves other people, such as shared items and conversations, raises questions about whose data it is and is often exported partially or not at all.
This is why an export can be technically complete under a legal definition and still omit the parts that made the service useful.
Incentives are one-directional
Services invest heavily in importing from competitors, because that removes a barrier to switching in their favour.
Exporting attracts far less effort, since it serves users who are leaving, and the requirement is satisfied by producing a file rather than a usable one.
Regulatory pressure has improved the availability of exports without doing much about their interoperability, which is the harder problem.
What makes an export genuinely useful
Documentation of the structure matters more than the format, because a documented file can be converted while an undocumented one must be reverse-engineered.
Stable identifiers, explicit relationships and inclusion of timestamps and original filenames preserve the structure that most reconstructions lose.
Testing an export by actually importing it somewhere, before it is needed, is the only reliable way to know which of these a service provides.