In a world under one uniform system of tightly-integrated government or closely allied nations at the nearly the same economic and technological levels, one can certainly do this.

Otherwise, yes, you are right, it's a grand pipe dream. Will all the nations contribute equally in terms of technological, financial and expert human resources ?

Besides, tribal, nationalistic thinking is very good for competition and speeding technological progress. It is what took US and Russia to space in the first place. Nations and civilisations try ten times as as hard if they know someone is competing with them.

Imagine if the Russians today had a Space-X competitor...