I think that's more of an economic incentives thing rather than any issue with this particular type of construction. You do a 1+5 building with soundproof doors

I think that's more of an economic incentives thing rather than any issue with this particular type of construction. You do a 1+5 building with soundproof doors, varied paint schemes for each unit, real wood flooring (not laminate), and so on. There's nothing about the building that prevents that. Developers have no incentive to do that stuff though because in most markets there's a big deficit in new construction and people will rent it, even if the doors are hollow and every wall is off-white.