Because it's embedded in their culture that hard work = success and success comes from hard work and staying hungry. They have to believe this because it's essential to feeling good about the sky-high inequality in various countries where this is worshipped (and more importantly - to feel justified in maintaining the unequal status quo).

The twitterb poster lll linked to is Vietnamese though

The people trying to insult him by saying his success is "just luck" probably include plenty of Americans. I think usually success needs both luck and hard work, so being lucky is not an insult but a given.

He's not American, hell he's not even from a western country.

It's not just a North American thing. Any entrepreneurial journey can feel very sisyphean, and if you worked hard on something for years, probably consciously making many sacrifices along the way, and then finally get rewarded for it, only to see the comments on social media diminishing your success as "just luck"... I can imagine they feel disrespected.

It's used by envious people to discredit other's success, and to justify why, conversely, they themselves haven't succeeded. It's very closely associated with having a "fixed mindset" and believing that people are merely a product of their environment/circumstances versus actually doing something about their own predicament.