If you want to build subways in less time for less money, the from-first-principles conclusion is that cut-and-cover is stupendously cheaper and quicker than tunneling.
But nobody wants to be the one who signs off on "long sections of major roads closed for months at a time over a period of several years".
You can’t really do multiple layers of tunnels with cut and cover. It also requires the tunnels to follow where you can actually cut instead of the most direct route.
> You can’t really do multiple layers of tunnels with cut and cover.
Sure you can. I ride such a line every week day.
True about the most direct route, but having uncontested right of way tends to make a big difference in travel times. And in grid layouts, where you want to go is closely aligned to how you get there.
But nobody wants to be the one who signs off on "long sections of major roads closed for months at a time over a period of several years".