Because the capitalists have very big guns and a monopoly on violence.
Its very easy to imagine a better world.
Its much harder to imagine the Scrooges in power willfully giving it up so that we can build that better world.
It's okay, I'll just use AI to vibengineer a viral basilisk that only targets those who violate the NAP.
It's not just the Scrooges in power. Previous attempts to actually make a better world often made a worse one, and not just because of capitalist opposition.