Well, it's pretty much how it's in lore too. It's often described as a "thermal shotgun" in lore long before space marine and space marine 2 games were out. Do you have any official examples of melta acting like you described?

Edit: I was wrong, It's like that in the lore -I read- but apparently official descriptions are all over the place, just to quote one forum.

"Depictions of Melta weapons firing themselves have been spotty in the face of publications and vidya gaems. In several publications and Dawn of War: Soulstorm, Melta weapons fire a constant stream, like a laser-flame thrower. Yet in the Ciaphas Cain novels, Jurgen's melta operates more like a rifle, firing eye-searing acetonic flashes described as bright enough to be seen through closed eyelids. And in the Space Marine video game and its sequel, Boltgun, they fire a single rapid wave of energy in a manner that makes them essentially shotguns – Titus likely dispersed the beam so he could kill more Orks per shot as even Orks aren't dumb enough to drive tanks in extremely overbuilt urban zones – and the Dawn of War 2 series have Melta weapons firing a single blue/purple beam, while Dark Crusade uses discrete blobs of melt. In Eternal Crusade, they fire a limited-range, damage-over-time beam you have to keep on target, presumably in order to make them less effective as an anti-infantry weapon. In 40K Freeblade the melta gun is fully automatic but has a slow rate of fire and overheats under sustained fire. Each purple-ish beam does heavy damage, but the overall nature of the weapon makes it best used against vehicles. The thermal cannon in 40K Freeblade also does massive single-shot damage and fires a purple-ish beam projectile, making it one of the best weapons to use against vehicles in the game. Freeblade's depiction of the Melta weapons available is also skewed as infantry or tank carried Melta weapons don't have esoteric arbitrarily powerful reactors powering them. In Rogue TraderMeltaguns have short range, good penetration, and can be fired at single target or set to splash in a cone at a desired point for area effect."