I think the general principle you're thinking about is only partially valid. Here is how I summarize it:
Mass production cannot happen without a factory. Factories can cost billions. But we do reuse the design of probes, especially the early probes in our history.
Research, design, and development have costs all their own. So I think there's a valid part of your principle that says, why design a new probe? We could just make multiple copies and send them everywhere they need to go.
Well, this is true for early space probes. A probe designed to flyby Jupiter would gather the same data from Saturn, Uranus, and Neptune.
And this is indeed how it was done at first. Voyager 1 & 2 were virtually identical, and I'm pretty sure the same is true of Pioneer 10 & 11 and the Mariner Probes. In addition, a lot of the really early probes to the Moon, Mars, and Venus were copies of each other. The Ranger series, for example, goes out to at least Ranger 8.
That does not mean we mass produce them. 8 Ranger probes is no justification for spending billions on a factory when they can just be assembled by hand.
But there comes a point when the same probe just won't gather any new data, because we've already done it! Time to design a new probe that gathers new data.
You may also be wondering about worlds unvisited, or recently visited, using entirely new probes. Why build New Horizons to go to Pluto when we can just reuse the Voyager design, for example? Why spend money on new development (and risk new failures) when we already have a valid design proven to work? The problem here is that the launch budget wasn't allocated for a 720 kg probe (Voyager's mass). Instead, we wanted a 480 kg probe (New Horizons' mass) so we could hurl it out to Pluto's distance really fast. Also, these days, designing a new space probe is NOT as hard as it used to be (thankfully).