Whatsapp themselves could easily solve this if they wanted... Just add a "backup to file" button in settings. Then add a "restore from file" option in Android.
Quite why they don't do this is a mystery to me - if a user loses all their chats in a phone migration, they're more likely to start using another messaging app.
I don't think Whatsapp gained anything by preventing this, if anything they gave people a little momentum to switching to another app. The one that clearly benefited was Apple, and I don't thing Whatsapp/Meta did it just to be nice.
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.
Quite why they don't do this is a mystery to me - if a user loses all their chats in a phone migration, they're more likely to start using another messaging app.