If you're an ordinary humanoid in a fantasy world, you can sit back and think of ways to stop things a mundane person can do. To stop things a magic user can do, however, requires knowledge of what a spellcaster is capable of.
Given the depth of spell lists, knowing that with any degree of certainty will be insanely difficult without extensive education and experience. Even if you've seen a Leomund's Indispensable Bivouac in action, do you know the exact limitations? Do you know if it has a floor, or how indestructible it is? Do you know how long it lasts, and other minute details? Only if you can cast it yourself, since you would need more than a brief encounter to know what you were dealing with. And let's not forget, most people who encounter adventurers die.
So for your campaign to have any kind of scenarios designed to deal with casters requires casters to be commonplace enough that their capabilities are fully understood. If your campaign is higher magic and spellcasters are everywhere, this makes perfect sense.
But often, in these sorts of discussions, you'll encounter people who consider their games to be lower to moderate magic. They don't want fantastic elements, preferring a more gritty "realistic" fantasy world without flying castles or teleport gates between cities, or golem city guards (things that a preponderance of casters would make possible).
In such a world, then, a lot of these "countermeasures" stand out because it's harder to imagine who has the detailed knowledge to conceive of or construct such things. The thing you're dealing with is communication- in our world, if someone has a cool idea, it can spread like wildfire.
In a world where travel distances matter, and you need riders on horseback or carrier pigeons to send messages, if one guy has a cool idea, it may be that only a few people ever hear about it. So even if a canny person creates countermeasures to one spell, it's very likely that it's going to be a one-off at best.
And even if a spellcaster figures out anti-spellcaster tech, you think he's going to just run around telling people how to beat his magic? Not very likely!