Because that's not how you do things in construction. If they are using it as a platform to drill piers off of, it makes no sense to build that bridge just to tear it down and build one in the same exact spot. You build the final bridge and you don't have to pay to build two bridges, nor waste the time. If they were doing a channel, they'd have to drain that area of the lake regardless, so they wouldn't need the bridge to work off of, nor could they even do that. They have no plans to remove that bridge. Saying differently just is showing you don't know what you are talking about.