GDPR is about privacy and security, not interoperability. It was (and still is) a very controversial set of regulations, so shoehorning in more requirements would have made it even more unpopular.
Interoperability seems a little difficult to enforce, like brokers that don't support fractional shares might need to start supporting them to import data from other brokers. Edit: I guess this example is poor but I just wanted to point out that schema/interop also involves data domain, validation, etc.
[0] https://ec.europa.eu/info/law/law-topic/data-protection/refo...
[1] https://www2.deloitte.com/ch/en/pages/risk/articles/gdpr-dat...
GDPR has its warts, absolutely. But I don't think that's a reason not to strive for even greater data protection measures for citizens going forward, especially if we amend the existing regulations to fix some of its unintended consequences.
[edit: typo]
Article 20 [RIGHT TO DATA PORTABILITY]
1. The data subject shall have the right to receive the personal data concerning him or her, which he or she has provided to a controller, in a structured, commonly used and machine-readable format and have the right to transmit those data to another controller without hindrance from the controller to which the personal data have been provided, where: ...