Because tunnels can go under buildings, roads and other tunnels with very little interference.

Bridges can also go above bridges, probably cheaper and safer as well

Underground doesn’t get affected by weather. I wonder whether simply having more tunnels side by side would help for servicing issues. Small, numerous tunnels instead of large tunnels.

Building wide tunnels is very hard and nullifies the main advantage of having the tunnel in the first place, which is easier routing.

Cost also vastly increases, likelihood of total failure as well. Big tunnels need big supports and big service ducts.