Contra von Neumann

It would be easy for an advanced technological civilisation to launch robot interstellar probes which would construct new probes from asteroidal material in each new system and launch them further while transmitting data home so that the entire galaxy would be explored at STL speeds in a manageable period so why has this not happened or has it without our knowing?

Poul Anderson thinks of two different reasons why not. In The Boat Of A Million Years, it is realised in advance that it will be impossible to assimilate so much data so no probes are sent that far. In Starfarers, von Neumann probes, as they are called, have been launched but their necessarily complex software is mutated by radiation before they have travelled very far so that they cease to function.

Anderson is to be commended for addressing this question, for thinking of different answers and for presenting them in entertaining works of fiction. This makes him, if not unique, then certainly exceptional among science fiction writers.