I wrote that "there are some areas where high speed rail makes sense in the US" and I think that answers your question of "why the coasts don't sport high-speed

I wrote that "there are some areas where high speed rail makes sense in the US" and I think that answers your question of "why the coasts don't sport high-speed rail". Specifically I cited the lower 48 density, not the US as a whole (including Alaska would be irrelevant), and I broke out California and the Northeast to show that they are more favorable to rail. I also mentioned that the Amtrak lines in Southern California and the Northeast make sense. Parts of the coasts do support high speed rail and as the video mentions, the coastal areas (California, Northeast, Florida, and even densely populated areas of Texas if you count the Gulf Coast) are building it albeit slower than many people would like.