Why has no one thought of this before?!

Obviously this is a terrible idea, but I'm gonna answer it seriously for the sake of dunking on it.

1. The amount of work. I mean, just astronomical. That's 1,650 miles of longitude this dude is talking about filling in; the largest earth-moving project ever was the Panama Canal, and it's only about 50 miles long. Plus, by comparison, it's essentially a one-dimensional line! This looks like it's probably in the ballpark of 500-ish miles from the current shore to the new shore, and two-ish miles from the surface to the floor.

2. Where would we get the land from? It's not like there's a pile just sitting around. I guess we could dredge the Pacific and truck it across to pour into the Atlantic? Take down the Appalachians and the Rockies? Bring down an asteroid into the ocean? None of that would be enough. In fact, nothing I can think of that we have access to could even come close to providing enough dirt (remember, we need 1,650 x 500 x 2 cubic miles of it!), even if we could manage to do it without destroying ecosystems or killing billions of people.

3. The people who have spent a lot of money buying homes and businesses on the current Eastern seaboard of the United States would probably have something to say about this plan. (Something loud and something very angry.) Besides, it would completely upend the shipping industry, the fishing industry, the tourism industry, and more. This would legitimately destroy multiple national economies, and that's before you even take into account the ecological disaster.

4. Sea level rise is already a major problem. So displacing a bunch of water in favor of dirt probably isn't going to help that too terribly much.

5. ...why? A lot of America is sitting unused or underused. If you were to clump all of the US's land use into discrete blocks, it would look like this: The area labeled "LAND?" on the ocean in the OP map is, give or take, the size of the current amount of land owned by the 100 largest landowning families, private family timberland, golf, and fallow land (meaning land used for nothing). This means that the area that the person in question is asking about is already essentially or literally being used for nothing at all. Before we start undertaking an ecologically-disastrous and fundamentally impossible project, we'd probably figure out ways to use that other land.

But there's more. The land that is being used is almost entirely being underused. For instance, take the "Cow pasture/range" section of the map; cattle account, by far, for the highest land use of any land use in the country. But the 28.2 million cows in America only need about an acre of land each; meaning that the 124.7 million acres of land they roam is about five times bigger than what they actually need. Most of the other production uses for land in the US (along with rural housing) are similarly sprawling because they can be; land is comparatively cheap, so there's no real reason to consolidate. If that changes, land prices will rise, and the people and companies holding on to underused land will discover that it makes financial sense to sell and reconfigure their businesses to make more efficient use of the land.

So calm down, Lex Luthor. The problem isn't that resources are actually scarce. It's that people at the top have a financial interest in underusing their holdings so that they can keep prices artificially high.