> It kinda advertises the defender position.

Which can be a point. What do you do if you're a foot soldier and such a bunker is on your way? First, you stop. Maybe your platoon tries to find a way around it, maybe you call reinforcements (direct fire) to take it out, maybe you set up a heavy machine gun to suppress it while you cross it. But all of this takes time, which is one of the main purposes of such defensive structures - to slow the enemy down. Interestingly, such a bunker will partially work in this role even if it's unoccupied, since making sure it is in fact empty will still take time. Now consider that there's almost a million of such bunkers slowing you down. You also have to consider the era, today they'd be an easier target than in the 1970s.