That's a fair point - I imagine the majority of players don't, and on some maps if you've adventured far enough away from your main in a Matilda or Valentine, it's literally faster to throw a satchel on it and wait for it to respawn than to drive back to resupply. In addition to this, I feel like a bit of a dick retreating in a tank for ammo if it's a tank others are likely to be waiting in line for.
It should be noted, though, that the patience to drive from St. Lambert to the German main and back again is nothing compared to the patience needed to hunt down the ammo truck if others have been driving off with it.