This is the old confusion about cover and concealment. Just because people can't see you well, doesn't mean you are in cover. Shooting through everyday barriers isn't particularly hard - you have to actively find a place where you are somewhat protected from bullets - the engine of a car or a barrel of water for example.
Gaining a bonus to defense requires an act on behalf of the target. In the described situation that's clearly not the case.
Concealment on the other hand is something that affects the shooter, in this case obscuring scenery. Bullets can still reach the target, even if it has to punch through a bit of metal or plastic first.
Well, first off, there isn't any modifier in the combat chapter for "Concealment" that modifies the attacker. If you're referring to the Visibility Environmental Modifier, that is very specifically referring to environmental conditions that broadly obscure vision, such as smoke, fog, or rain. The environmental modifiers don't deal with terrain/barriers at all.
And the cover rules don't actually require you to be taking cover behind a particular type of barrier. A paper wall and concrete slab will provide equal bonuses as far as the cover bonus is concerned for defense. The only thing it cares about is amount of body coverage. And it does bring in the rule that ties cause the attack to hit now (instead of miss) but the attack was
through the barrier.
Now, I do agree that "Taking Cover" does require a willful act, which is why I suggested that just happening to be behind cover does not provide the same level of protection. But actively taking cover does not mystically make the terrain any more difficult to shoot through, and neither does
not having taken cover make it easier to shoot
through. Clearly, if you are going to apply some sort of penalty to the attacker for the target being concealed by terrain, it should apply
any time terrain blocks sight... which is at least partially what the Take Cover action provides. That's why it gives a bigger bonus for being better/more cover. Which brings us full circle back around to why I would consider physical barriers to fall under the Cover rules.