Why Not Tunnels?

Are there any civil engineers or other similarly informed individuals in the OT who can talk about why tunnels aren’t being discussed as an alternative to a new bridge to solve Baton Rouge’s traffic problems? I’m sure it’s been considered and eliminated as a possible solution for good reason, but I’m curious to know why. Prohibitive cost comes most immediately to mind, but....

The George Wallace Tunnel (GWT) in Alabama was finished in ‘73 at a cost of $50M, which is $293.7M in today’s dollars. The GWT is 3,000 feet long. From one side of the Mississippi River to the other where the current bridge is built is about 2,700 feet. What I know is that we have many different considerations here that they may not have there such as levees and boat traffic depth which would likely drive up the cost, but would those things drive it up THAT much? $293.7M is about 1/4th the amount of the most recent bridge estimate I saw of $1.2B. For that amount, without any more information, it seems like you could build multiple tunnels for the price of a bridge. Also, I’m sure the Corp would have something to say about a Mississippi River tunnel.

I’m sure I’m missing something (if not multiple things) here. I know nothing about engineering, just curious. Please school me. Thanks.