WhatsApp isn't made by a single indie developer, and wasn't just released by that single indie developer a month ago. If it can't sync between Apple Phones and Android, then WhatsApp has intentionally chosen not to fix that and instead spend many millions of dollars on other stuff. It is on purpose.

https://en.wikipedia.org/wiki/Conway%27s_law at work.

Never used WhatsApp, but is it related to Apple policies? I just ran into something reminiscent of this with Microsoft's Authenticator app - runs on both platforms, supports backup and restore, but apparently on Apple devices it's only able to use iCloud for backup rather than an associated Microsoft account.

The net effect is that if you're switching you have to redo 2fa for all accounts where you haven't separately backed up your TOTP codes.

What makes you sure it's not on purpose? I don't disagree, I'm just curious about your statement.

Why would WhatsApp want to make it hard to stay on their platform?

Thats a different question but 1) they know and 2) they can fix it fast therefore 3) it is on purpose