An analogy to depict it:
Plumber: "You have a dead rodent stuck in your drain line, we need break ground and replace a major section of piping."
Homeowner: "How do you know it's a rodent?"
Plumber: "Your line is backing up exactly the same way hundreds of houses in this area have been. When it started happening 12 years ago, the plumber on site dug up a line an found a dead rodent right next to the stoppage area and there was a hole in the drain line where the rodent MUST Have eaten through."
Homeowner: "So, you finding rodents at all of the houses that are backing up?"
Plumber: "No, but they're all backing up exactly the same way, so we just skip the expensive investigatory work and replace the piping."
Homeowner: "Did you ever personally see a rodent in any of the line you replaced?"
Homeowner: "Well, surely you have some pictures of rodents your coworkers found?"
Plumber: "No. But i can get you a picture of the one the found at they first house 12 years ago."
Another analogy to depict it:
Take a River..
A Whirpool - Exists as a result of the Shape of the river and/or water flow. You can not remove it from the river to examine it independent of the river. And if for some reason you could construct a mechanism keep it swirling while removing it , it would still no longer be the same whirlpool as existed as part of the river.
A Beaver Dam - Exists as a result of beavers. Only beavers can make a beaver dam.
A Pest Infestation - Exists as a result of climate, or surrounding conditions. similar to beaver dam.
Of these three, only the Pest can be removed and isolated from it's habitat and examined while still in it's genuine state. The Beaver Dam and the Whirlpool pool are no longer genuine once removed from their habitat.
I believe a lot of people are suspicious that viruses are more like a Whirlpool rather than a Pest Infestation. And they're very skeptical when your answer to "show me it" is something like "they did isolate the pest from it's habitat at least two separate times back in the 40's/50's or whatever"