It takes a decade and many billions of dollars to dig a single tunnel in most cities in this country. So best of luck with your plan..
Have people lost the will to think big because others before them have failed, or do people assume that they will fail because they don't think big.
I guess the asset-light strategy works sometimes (a lot of cities are regretting mass transit investments post-pandemic), but it sure doesn't seem to lead to better quality of life.
What gives you your understanding? Mine is from multiple sources at the top of Google for the query "price of tunnel versus price of bridge"
Engineering costs its a matter of volume, what kind of ground you need to dig, expertise and experience. If you can get something like the Boring company up, costs would drop down.
And engineering costs don’t account for externalities of having a bridge full of noisy cars hanging there, polluting in every modality: view, noise, smell, splitting the neighborhood, reducing walkability, destroying the property value.