peter Because there is no edit mode in the "Add Songs" dialog. You are not supposed to edit songs in that dialog.
No one expects to edit from that dialog but we do expect to have the same selection capability of select all just like in playlist and song mode as it’s a very important function and it’s not there if you go hunting for songs to add to your playlist. If you leave playlist mode and go to song mode that same screen does have select all capability but if you enter this same song screen from the playlist + mode it does not have this capability, so then the interface appears to have more different screens which complicates it unnecessarily , perhaps include the same edit button and the select all function only in this case so it feels consistent and allows us to add songs more efficiently when doing it from a playlist we are working in, that way it’s less confusing and much more convenient and logical to use.
I find using song mode, edit, add songs to playlists /queue mode functions confusing as I don’t use queue mode why is that function button displayed as an option as I have turned queue mode off in settings , I can never work out which button to press when selecting a destination , for that reason I always create or enter a playlist mode first then I use the + add to playlist which saves me time not to have to enter where my destination is but it also screws me cause I don’t get the select all function.
Suggestion is to make queue button disappear in song mode when queue is deselected in settings , and add the edit button in playlist + mode with the select all function only to be there and any other function that might be relevant in that mode.