I would chalk it up more to the slow erosion of social expectations and standards, the proliferation of cell phones and social media, and the consistent push towards individualism and away from family/community values.
You can't just exercise, you need to be participating in some mudbeast games or run marathons or lift 5 times your body weight. You can't eat healthy, you have to religiously follow the latest fad diet and preach it while you are at it. You can't just work hard, you have to embrace hustle culture. Your kids need to be constantly exhausted from being in every sport and extracurricular you can sign them up for.
Oh, and your efforts on all these axes must be quantified, tracked, and gamified. Preferably by an app you pay a subscription fee for. And if your psyche rebels against all this you need therapy and/or pills. Because otherwise you are simply not competitive.
And that's the key: competitive. People don't have friends here. They have the Joneses to compete with.
A common refrain heard in Russia when it switched from Communism to Capitalism back in the 1990s was that people stopped talking to their neighbors. Before the switch everyone was equally poor so there was no envy. There was little pressure to work hard, so people had more time to socialize. The change was abrupt enough that everyone noticed.
I think you could blame the loneliness issue on excessive individualism, but I wouldn’t blame it on competitive, hard-working culture. Especially because there are many places in the US which absolutely do not have a competitive culture yet still suffer from loneliness.