Even on the rare occasion when people spot for arty (not "Enemy tank detected!", that we get plenty of), they typically target a fast-moving tank in the middle of nowhere. By the time the arty gunner has had time to adjust the sights, there's no tank, just empty ground in the middle of nowhere. And typing into text chat "bad spot, empty terrain" has no effect at all.
Of course, this works both ways, so if you spot for arty and ask "arty, is the spot good" there will be no reply.