S21 of a integrated TL versus a metal interconnect

If we are trying to connect 2 circuit blocks on the same chip through one of these 2 mediums :
i) metal interconnect
ii) integrated CPW / microstrip TL

Why is there less signal attenuation in the case (ii). Physically, the only difference between the 2 cases is that the ground track is explicit for the Transmission Line. In the CPW the ground consists of 2 metal lines adjacent to the conductor and in the microstrip we build the groud plane

How does that change the amount of signal attenuation?

Metal inteconnects don't touch to the substrate, they are placed on different layers such as epitaxy, SiO2 etc.So, metal interconnections are not purely transmission lines and therefore they can not guide the waves as well as real transmission lines...

Thanks for the reply! But I am not completely convinced by the answer.

For a CPW i would consider to use the top level metal to increase distance from the lossy substrate.

For a metal interconnect too I would use the top level metal because the top level metal is the thickest and has least resistance.

So, neither touch the substrate. The loss mechanism in either case seems to be the finite resistance of the metal track. Am i missing something?