Well that would be because Blizzard nerfed scaling for groups the day Delves were released.
And judging by how it turned out, they nerfed it without testing it in comparison to solo.
Edit: As others have stated it can be done solo and has been done solo by most roles already. It is a matter of gear and player skill. Why should an ilvl590-600 be able to clear recommended ilvl640 content? That’s the actual issue.