Why WhatsApp photos are saved to camera roll and not to a separate folder?

All photos are saved to the camera roll. What makes WhatsApp so special?

BTW, WhatsApp is not an Apple product, best you tell the app developer what you want.

You can create a folder and move the photos there.

Create a new album. Tap Albums, tap +,enter a name, then tap Save. Select photos and videos, then tap Done.

Add items to an existing album. While viewing photo thumbnails, tap Select, select items, tap Add To, then select the album.

This is the default:

The Albums tab includes albums you create yourself and albums that Photos creates for you, based on photo or video type. For example, photos you take with the front-facing camera are automatically added to the Selfies album. Other albums that Photos creates include Favorites, People, Places, Live Photos, Videos, Panoramas, Slo-mo, Bursts, Screenshots, and more.