Most of these tunnels are being built in heavily populated areas where surface space is at a premium. You'd have to tear down thousands of apartments and office

Most of these tunnels are being built in heavily populated areas where surface space is at a premium. You'd have to tear down thousands of apartments and offices to make any meaningful progress on a cut and cover tunnel inside a city unless the tunnel is exactly following existing roads. That would be both very expensive and far more disruptive than just a little traffic