Original response below:
That's really a WhatsApp product issue, not an OS issue. There's some hints of an OS issue, because Android lets WA put a backup file on the 'sd card' that you can transfer across to a new (Android) phone, and iOS doesn't (or didn't), and with cloud backups the different OSes both tie into their own clouds.
But the main issue is the WA iOS app and the WA Android app have different schemas for their on device database, which makes it not so easy to move. Maybe that has changed since I stopped working there, but that was the biggest issue with a switch platforms feature that I was aware of. It's a lot of coordination for a feature that most users are never going to use, and if they do use, likely aren't going to use it more than once. When I recently got a new Android, I did see there's a new transfer data flow for at least Android to Android, so maybe there's hope for cross OS data exchange in the future? It's also helpful that there's only two relevant platforms now, instead of 7 (s40, s60, blackberry, blackberry 10, windows phone are all dead)
I switched to iPhone 15 Pro recently from Android and after trying to import my data from Android couple of times and iOS failing to import WhatsApp specifically, I had to resort to buying third party software to perform the message transfer via a Windows laptop.
Bear in mind the import process took like 3 hours each time and I had to keep both phones close to each other, couldn't use them while importing and had to keep power supplied to them.
After about 10 hours of trying, I gave in and put 100$ towards proper third party software to transfer my messages. This is ridiculous, as I have Google Drive on my iPhone with my WhatsApp backups from my old phone, however for one reason or another these backups cannot be utilised by WhatsApp on iOS.
Moving between two Android phones "it just works".
The built in services transfer failed as did third party software.
Honestly I can’t tell you much about transferring the other way, but interoperability is definitely not seamless in this respect.
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.
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.