“Capitalism is the best economic system that we have had so far”

At the risk of sounding like a tankie: a huge factor why none succeeded is that the west and capitalism did their best - ranging from economic warfare to the CIA murdering popular leaders in Latin America - to make them fail.

I don't disagree that modern socialism has yet to be achieved, but to claim no socialist systems have ever existed is a very settler-colonialist take. Even Marx himself knew better:

"to look beyond the Middle Ages into the primitive age of every people—and this corresponds to the socialist tendency, though these learned men [Georg Ludwig von Mauer, known for his studies of early German communal society, and Jakob Grimm, the philologist and cultural historian] have no idea they [the “primitive” communal forms] are connected with it [the socialist tendency]. And they are then surprised to find what is newest in what is oldest." source