That's not necessarily simple though, because some companies are shady and ass about it. And then if you want to pursue the matter, be ready to face significant effort needed to file a proper complaint in the correct channels for that.

Fwiw, GDPR has a forgotten clause that data exports need to also be provided in machine-readable format for easy service migrations and promoting competition / reducing vendor lock-in

The only reason I can imagine why nobody implements this, is that they like the status quo where it's hard to switch to their service, but equally hard to switch away. Once this becomes commonplace (e.g. because vendor A makes users aware it's a thing by implementing it for migrating from vendor B), vendor B will "retaliate" and A will have to truly be better than vendor B to retain said users

That is all to say, this might yet totally be a viable and legally unblockable business avenue for a third party to initiate. I'm just not sure there's enough demand to warrant (read: pay for) the effort involved. Maybe if you can have an LLM read the api docs of the service where you want to do the import and autogenerate a mostly-working version?