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".

Tunnels are a more costly than bridges, especially if you can't just cut and cover.

Hence me specifying roof-top parks. You know, with lots of waterlogged soil and trees. The static load of that is significantly higher, but of course heavy trucks cause lots of dynamic load, which is another difficult issue.

It wasn't mentioned in the article, but I suspect there was also pressure to keep as much of the highway open for as long as possible. Putting in a tunnel, or changing the road to a bridge would close a major shipping route for months, whereas putting in an overpass can be done with staggered lane closures.