Is there any technical reason that it has to send your notification data to Google and Apple or is it just to get more data on you?

the API that for example Google Firebases provides (most used, as it supports ios and android), is basically "send a notification with following content to this device".

Which is very simple to implement. as it's just fire and forget. But you send the actual data to Google in this case.

There are way to do it differently, for example how signal does it: They send a silent (e.g. invisible) notification to the device, which has no data in it.

That notifiation tells the app to check for new messages.

The app will then fetch messages in the encrypted way as it always does, and displays a notification if needed. No need to send actual data through the notification service (other than the metadata, that notifications should be pulled)