It's not as simplistic as "penis size" it's part of a culture where being manliness and independence is valued. People are also told we need to be tougher and how we are losing what makes America great.
For example "In the trailer for "The End of Men," Tucker Carlson worries about "the total collapse of testosterone levels in American men,"
Yes it is and according to what you linked to "According to Lokeshwar, potential causes for these declines could be increased obesity/BMI, assay variations, diet/phytoestrogens, declined exercise and physical activity, fat percentage, marijuana use, and environmental toxins."
Tucker Carlson is using it as a very thin excuse to basically make a series about how men need to be more like a particular cultural concept of men.
Here's the trailer for "The End of Men" nowhere does it mention that study or a testosterone drop. Maybe the show itself touches on that but in my opinion that would only be so people like you could justify what is basically a propaganda video for right wing masculine culture.
For example "In the trailer for "The End of Men," Tucker Carlson worries about "the total collapse of testosterone levels in American men,"