I think it's because they think "individualism" means "I do whatever I want and take whatever I want with no consequences, and I'm a better person because I do those things." Which I don't consider individualistic at all, it's more that they just want to be thoughtless assholes about everything and not worry about social or economic consequences for it.
It's similar to how capitalism can get recontextualized into something meaningless and shitty by Objectivists. When capitalism works, it can almost seem magical, because it's basically a greedy algorithm that can often produce a net good for most of the population, based on individuals looking out for their own interests. Hence notions like "greed is good" or "selfishness is a virtue". But the whole point of those notions is, they're built on an assumption that those things lead to a net good for everyone involved. To the degree that capitalism works, it's because it allows wealth to be created, which is a net good for everyone.
The purpose of capitalism as the core of a civilization is wealth creation, where people exchange things they don't want in return for things they do want. All participants in the transaction are better off than before in their eyes, but nothing new has been created. Money and goods and services have simply changed hands. Everyone's better off than they were before, but no new goods have been created, and no new money has been printed. It's like creating something from nothing. If any argument can be made in favor of selfishness being a virtue, this is it- it's a virtue because in this context it supposedly leads to a better world for everyone, not just individuals.
But I've known plenty of right wingers who claim to draw inspiration from libertarianism, or who claim to have a special understanding and reverence for capitalism because they're Objectivists. And yet, they never talk about wealth creation. To them, selfishness is a virtue simply because that's the world they want to live in. These are the temporarily frustrated millionaires living in compounds in Wyoming, mooching off the government while complaining about urban welfare recipients. They think Objectivism or libertarianism or their religion gives them cover to go around thinking that greed and selfishness are actually good things, without ever embracing or even understanding the foundations of those beliefs- that they're only good things in the context of maximally efficient and rational markets, and only because they supposedly lead to wealth creation (everyone getting richer in some sense, not just individuals getting rich at the expense of other individuals). But they're not in it for the wealth creation- to them, capitalism isn't good because it creates wealth, rather capitalism must be good because they think it means it's OK for them to be selfish assholes.