I think they did the "attack through wall tactic" to make it a little bit faster because who wants to walk around entire wall just to attack a enemy when you can just snipe them by attacking behind a wall.
More likely because checking for obstructions was too slow in the NES Fire Emblem, and it wasn't a real issue, to the point where it's arguable whether it's a bad thing or not, so it became a game mechanic in the long-term. --- *(int *)nullptr = 0; http://me.veekun.com/blog/2012/04/09/php-a-fractal-of-bad-design/
Being able to attack through walls lets there be more hazards in indoor chapters though. Half the indoor chapters across the whole series would be pretty boring or just wouldn't work if units couldn't attack through walls.
I'd take more interesting map challenges and set ups before limiting it with realism.