High-speed rail is very limited in the US compared to several other countries, like China, Japan, France, Spain, Italy, and Germany.
It has to do with most of these countries being far more compact than US. ...
List of states and territories of the United States by population density -
List of countries and dependencies by population density
I'll use per square kilometer.
China: 149, though most of it's HSR development is in the populous eastern half. The western half is thinly populated mountains and deserts and plateaus. That suggests a figure near 300 where the HSR lines are.
Japan: 337, South Korea: 511, Taiwan: 652, France: 118, Spain: 92, Italy: 201, Germany: 223 -- high-speed lines are spread over much of the territory of each nation.
Now for US states. The Northeast Corridor is the closest that the US gets to HSR. The states that it runs through: MA 336, RI 394, CT 286, NY 162, NJ 470, PA 110, DE 187, MD 238, DC 4251 -- the best comparison is states mostly near the line, like CT and RI, and to a lesser extent, MA, NJ, DE, and MD. Most of NY's land area is upstate NY, and most of PA's land area is away from its southeastern corner, where the line is.
So the NEC has plenty of density. Let us now look at the peripheral states, ME 16, NH 57, VT 26, VA 81. Going to Florida: NC 79, SC 62, GA 68, FL 145
What I like to call Greater Chicagoland: IL 89, MI 67, OH 109, IN 71, MO 34, IA 21, MN 26, WI 41 -- a bit stretching it. Going further westward: ND 4, SD 4, NE 9, KS 14, OK 22 -- Not very suited for HSR. The mountain states are also thinly populated, and it's on the west coast that it becomes better for HSR: CA 97, OR 16, WA 41. Finally, TX 40.
So the density argument does not hold much water.