"Why replace them at all? If they serve no function, just remove them!........"
Aesthethics. Lots of visible structure with that stuff removed. Folks want a nice smooth tunnel to drive in. Ugly concrete and metal makes folks nervous when they're underground.
I don't drive thru tunnels looking at the ceiling. I just want to get to the end as soon as possible and live to tell about it.......
I used to work in a place that was a recycled factory, with loads of ducts and pipes running all over. Someone got the bright idea to paint them all bright, cheery colors. It actually looked good!