Tunnels are even more unbelievably prohibitively expensive than this. Which makes sense, because typically one of the cheapest and most reliable ways to build a tunnel is actually pretty much the same thing they're doing to build this bridge: cut and cover. You dig a trench where you want the tunnel to be with conventional excavation, put the road and tunnel in, then cover it back up with material over top. Here they just get to skip most of the initial excavation step, and go straight to "cover".