iPhone is popular and defaults matter. People are not going out of their way to install iMessage.
For the bigger groups, or groups with older people, we use WhatsApp since there might be an Android user or less risk of having to exclude someone because they switched to Android.
Edit: it seems like the way to ensure sharing photos/video at original quality in iOS/macOS is to select the media in photos app, and then share, and then select “copy iCloud link”.
> When shared, photos are reduced to 2048 pixels on the long edge, except panoramic photos, which can be up to 5400 pixels wide. You can share GIFs that are 100MB or smaller.
I do not see a material advantage of iMessage over WhatsApp now.