Comment by londons_explore

I can see why they don't want to let it be possible to export from whatsapp and re-import into a competing chat app. They've gone to quite some efforts to encrypt databases etc. to make that hard.

Allowing cross-platform transfers means they can't use the platforms secure storage features to achieve this - instead they need to write server side code to generate some per-user key, and some DRM-like scheme to validate that only the official client app is requesting the key to decrypt a backup.

I can see why they want to just leave all of that to the platform.