Any tsunami video I look at barely looks like a 20 feet wave hitting shore. But wikipedia tells me there have been dozens of 100+ feet tsunamis even in the last 10 years.
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Most earthquake-related tsunamis happen at depth below surface of the ocean and produce a huge bulge over many kilometers, so aren’t easily seen because we judge height by differences with surroundings, and the surroundings are all rising too. the slope is low when a 40 meter high bulge is spread over a distance of many kilometers, something like 3-5 meters per kilometer is only a shallow hill that won’t look like much if even noticed at all.
When they hit land, though, the front of the wave might come in as a few-meter high wall (water slows down and piles up when it reaches shore), but the 40 m high peak is still out in the ocean a few kilometers while that is happening. At a place on the coast, the front wave is followed by more and more water, so the water keeps rising (just keeps coming and coming), until the top of the wave makes it all the way to shore, and that generally takes several minutes.
There are some tsunamis that are produced by punctual (very localized) events like a landslide into a deep bay, or a volcano explosion. Those will be very high at the point of origin, perhaps more than 30m/100 feet (it happens from time to time; Alaska gets one about every decade or so in one of its many fjords) and could keep very tall if confined to a valley (cannot flatten out by radiating away from the point of origin). We don’t have films of such events because they are random and rare, and mostly happen where no one lives so no one was lucky enough to be filming while also not in the valley and hit by the wave itself. The destruction of trees up some one hundred feet along the valley walls though is pretty good proof that a 100 foot wave happened there, even if no one saw it (sometimes people do see them but no one has filmed one yet, as far as I know).
Closest film I have seen of a punctual wave being formed is from glacial calving (not even close to 100 feet high though) or when landslides hit man-made water-retention structures. You can find films like that on youtube and the are spectacular enough even if not worldwide events. Also flash flood from a dam breaking, but although similar to tsunami (and maybe even the dam broke because of a tsunami in the reservoir), they aren’t really tsunamis.
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