If there are many metal layers in a process (let's say, 10 or 15), lower metal layers are thin and highly resistive, and upper metal layers are thick and low resistive.
That's why top metal layers are used for lateral routing of source and drain nets.
To get down to active devices, one has to go through all other metal layers, vertically.
That's why all metal layers get used: top layers - mostly for lateral routing, lower metal layers - mostly for vertical routing.
The same is true not only for source/drain nets. but for other nets as well - gate, power, clocks, etc.