Marxists are so caught up in the central planning mindset that they think capitalism is susceptible to their permutation of the "communism has never been tried" argument: pure capitalism has never been successfully implemented as the economic policy of any government.
But while socialism requires such miraculous conditions that even people's greatest attempts fail so hard as to be discredited for not even trying the "real thing," the observable forces of market interaction are always at play, no matter how much the government stifles and suppresses them.
At every possible opportunity, real capitalism is not only attempted but succeeds. It's just that central planning governments are inherently impediments to that process.