No logical reason, the implication is they use m-drives not reaction drives so it would be entirely possible for them to vary their thrust. The issue will be the very "simple" brain that is implied. With a robot missile chassis you could fit a more intelligent brain and give it an auto-pilot. Then you could give it verbal orders and it could work it out itself. Probably too expensive and a bit involved for a merchant though.Further thought... is there any logical reason missiles couldn't be programmed to hang around the launch ship until a group is gathered, THEN be sent on their merry way so that they arrive all together (whether grouped as one salvo or many)? There may be control limits, although that's probably a function of the ship's electronics. That is, a standard missile launcher on a civilian ship launching stock missiles likely does not allow this tactic, but a ship with military grade electronics probably could. Or you might pay a bit more for a fancy missile with a few tricks up its sleeve. Or the ship's technician might be able to do something about it.
If you want a lot of missiles in the air the Container Missile option in Companion gives a surge capability, but not significantly more than a triple launcher and no reload capability either. It makes more sense for boats.Clearly NOT a good tactic to do too close to something that can shoot at them, but at range (especially Long range) I could see it as being practical.
It starts to get expensive as well.