Standing on one foot, the government acts in the workers' interests, that is the entirety of socialism, and the rest is its interpretation.
Why the workers' interests? Why not humanity's interests? Everyone's interests?
In part, the reasoning is, I think, along the same lines as #BlackLivesMatter. The intent of this slogan is not that only Black lives matter. Instead, everyone already agrees that White lives matter, but no small few seem to believe that Black lives do not matter. White people do not need a political movement to protect their lives because their lives are already protected. Black people do need a political movement because their lives are presently not protected by law and custom.
Similarly with socialism, at least in part: all humanity does matter, but billionaires and their supporters and enablers do not need a political movement to get the government to act in their interests. The government already acts in interests of the billionaires, but acts against workers' interests.
But in part, socialism is dissimilar to BLM. I hold as an article of faith that white people's interests are not fundamentally opposed to black people's interests, however presently entrenched the opposition. In contrast, billionaires' interests are fundamentally opposed to workers' interests. The government cannot act in workers' interests without acting against billionaires' interests. The only final resolution to the conflict is to eliminate the billionaires.
Happily, it is at least theoretically possible to eliminate the billionaires without killing anyone: we need only take away their money, not their lives. The billionaires might fight to the death to preserve their power and privilege, but that's their choice, not ours.
A note on capitalization: I use the capitalized terms White and Black to denote socially constructed racialization. In this sense, White interests are fundamentally opposed to Black interests. I use the uncapitalized terms white and black to denote the physical characteristics that we usually use to socially construct race. I personally am white, but I do not see myself (or I do my best to not see myself) as White. Also, the social construction is not symmetric: Whiteness is intrinsically racist, but Blackness is not, because Blackness is a response to White racism.