Because socialism requires a strong capitalist base from which to transition.
You can't transfer ownership of the means of production to the public if there is no means of production. Socialism is intended to be the "end game" of capitalism.

Socialism is a nice idea but only really works in a post scarcity society (something marx himself said would require a capitalist one to manage to get too). You can;t have a classless, money free society if someone has to work in the fields while someone gets to study art, or science and do no real work their entire lives.

We're on the verge of that now with automation (assuming we don't overheat the planet, kill ourselves in wars or what have you).
When 100% of mankind's basic needs (aka all the shitty hard manual labour jobs) are done by automation - then a true socialist society will be viable.

Universal basic income is the first stepping stone to that. Once you;re paying enough to live simply freeing everyone from menial labour - then you can begin distributing the fruits of automation even more equally.

Eventually (I hope) we get a star trek like society with no money, where everyone is free to pursue their passions, of if they wish sit on their ass playing halo games all day every day.

Again though, why is it one extreme or the other? It's moronic to dismiss the whole concept of socialism just because it's failed a few times. It's even more moronic to refuse to apply sensible and necessary restrictions to capitalism (and capitalists) via governmental intervention because capitalism worked better at a specific point in history.

That intervention is taxes, the idea that it is unfair to heavily tax the rich to serve the poor is just unfathomable to me. Those rich are the lucky few benefiting the most from the fruits of society, they have a responsibility to put back into it not just hoard it.

In an ideal world you wouldn't need a government to take it via taxes, all wealthy (and I'm talking people who tens of millions a year) would go the Bill gates route and use it for the betterment of mankind freely.