While FDR was popular, he was a social democrat. Welfare under capitalism is not socialism, it's just welfare, and the welfare the US received was largely built from colonial holdings and imperialist plunder. Socialism isn't just having decent welfare, it's an entirely new economic system where the base of society is determined by social ownership of production and distribution. The only reason FDR was even allowed to pass these social reforms was because, at the time, economic unrest was high and workers could look to a largely unaffected and growing USSR for inspiration. The New Deal was a concession made to an organized working class, and the source was largely imperialist super-profits, ensuring both capitalists and workers could domestically be happy (assuming you were white). They exported the contradictions of society to the global south.