I think it has something to do with how retrospective it is and how widely available the material is otherwise.
For instance I think Hatful of Hollow has a right to be considered as an album even though it wasn’t recorded as one - all the songs were recorded over a period of about 15 months and a lot of the songs and almost all of the recordings are unique to it. It was released shortly after the songs were recorded. It’s clearly different from an end of career ‘greatest hits’ type record.
Lots of ‘albums’ are recorded over a long period of time and include lots of tracks that have been previously released but we still think of them as albums.
In the end a lot of it comes down to how they are titled and presented. Phases is definitely presented as an album to my eyes.