Because Directly Placed Templates represetn weapons that are unaffected by cover. Flamethrowers were used widely in several wars to clear out tunnels, pillboxes, etc because they afforded no cover to the enemy. The fire burned not only flesh, but consumed tohe available oxygen in the area, as well.
Shotguns are comprised of hundreds of pellet/flechettes that are designed for coverage over a wide area, ensuring that the targets are at least hit by some shrapnel. Further, the weapons are, already limited in distance (7" - 10"). Balance that with the fact that they ignore cover, and you still have to use good tactics to get in and be able to use it, effectively, as you should have to do with any weapon.
The only cover from these is LOS. You can't hit what you can't see.