Because it's convenient. The alternative would be something like flatpak's portals, which funnel everything through a few standardized dialogs; but how would you for instance use them to implement a media player application which scans for mp3 files, reads their tags, and presents them on a list? A "select a directory" portal dialog either would not allow for a recursive scan, or risk a non-technical user selecting their home directory, and either way would be a strange interruption in the workflow. (I understand, however, that Android has done precisely that for removable SD cards...)