> Most authors don't even bother building in reasons why that can't happen, its just that no-one thinks of doing it.

Is it really necessary to spell out that a civilization capable of building a FTL starship is also capable of building a sufficient defensive technology?

> I daresay such a civ is not doing it for conquest, but to discover and explore. If the urge is militaristic, then such a civ would turn on itself long before the project succeeds.

I think this is much more complex. Consider a civilization so advanced that this is children's toys for them - and then that civilization falls. Then, on the scraps of it, a new imperialistic faction is built and they go for the conquest; they don't even need to understand the magic that powers their warships. I recommend you to read the full Foundation series in chronological order.