The Japanese had long since built immense concrete sea walls (and other barriers like thick clusters of tough, fast-growing trees) throughout the length of their eastern coastlines, and it didn't help enormously during the 2011 earthquake/tsunami. The sheer energy of the waves simply pushed the water over these barriers and killed thousands of people while destroying entire cities.
Another reply here alludes to the fundamental problem with tsunamis by mentioning a phenomenon that's more like the ocean suddenly growing deeper. That's not exactly right but it is close in a way. Tsunamis are vast water displacement events and completely unlike normal large tidal or wind-caused (storm surge) waves because while you might have a storm wave of, say, 15 meters height and a tsunami of the same height, the The tidal/storm wave has much less run-up mass/volume, it's usually just a bit more than what you get right before your eyes. The tsunami on the other hand has a lateral run-up mass behind it that stretches back for as many as dozens of kilometers if I remember correctly, and all of that mass has to keep moving forward until it exhausts itself. Thus when the wave first hits, that's just the very beginning of all the destructive power it brings. A whole vast freight train (so to speak) of surging water mass, with all the displacement energy that caused it built right in, still has to keep moving forward until it expires. This vastly destructive process can take a while to complete itself.
You see why this is also a problem when it comes to sea walls too? If you have a 15 meter concrete sea wall and it gets hit by a 20 meter storm wave, the wave might sort of cross over its top and flood a bit on the other side, but otherwise the sea wall does its basic job. But if that same sea wall is struck by a tsunami of even slightly less than its height, the surging lateral mass of water behind the initial wave just keep pushing forward tremendously until it heavily overflows the wall.
If you watch videos of the 2011 tsunami, and especially videos where the wave actually hits barriers and then overflows them completely, you'll see the above effects in action. Terrifying stuff and very unique to tsunamis, which, I repeat, are completely unlike any ordinary large wave.