To answer why people would push to get so fit, you have to explain the emotion and motivation behind it. Simply saying it's healthy or makes you strong only extends the question.
Partly they do it because of the emotional reward from exhaustion, progressing overcoming challenges, feeling stronger or more capable, approaching an ideal appearance, comparing themselves to others, and maybe a couple others. I believe all of those things make people feel good and want to continue.
You also have to explain why now compared to a few decades ago, though, which the above doesn't explain.
I think the main reason is that the American environments surround us with gluttonous and unhealthy food and behaviors that, if we didn't work to make ourselves healthy, we could too easily slide down a slippery slope and become. The more junk food and labor-saving devices around us, the greater the temptation, therefore the greater need to resist, therefore the greater enthusiasm to work.
In principle you could test this hypothesis. It suggests that in communities with healthy food and active lifestyles, you'd find less extreme fitness and more regular fitness. In communities with unhealthy food and sedentary lifestyles, you'd find the people who pursued fitness would pursue it more extremely.