Funny, you'd think 40K players of all people would appreciate a force multiplier.

Most game systems I read do not adopt such a simplistic "kills = victory!" mentality. Outnumbering opponents, having supporting units, having morale boosts, being better drilled, being so bloody psychotic you refuse to fall over... these and countless others are used as justification for granting combat bonuses beyond how many people you can hit with a big stick.
The reason I used Gaunts vs Marines is because it highlights what is so fundamentally
wrong with this game. Look a little deeper, if you will:
30 Gaunts. 30 models that exist
solely to engage and destroy enemies in close combat.
10 Marines. 10 models that have ranged ability, and fill an 'all round' role.
In theory, these units are balanced. In theory, assuming equal point values, victory would boil down to who went first, or who charged. The Gaunts should take losses from the Marine shooting, but then have the raw combat power to make up the difference in an orgy of melee combat.
Thing is, as my last game as Necrons vs Tyranids proved, it doesn't work that way. My units bounced charges by standing there and relying on high toughness and great saves to carry them through, then bitchslapped their weak, nigh-armourless opponents. Despite my units not being meant for close combat, I was able to win, or at least draw, against a dedicated assault unit. Why? Because only kills matter.
Force multipliers would not automatically swing the game in favour of the horde. Remember, putting all your eggs in one basket has the potential for catastrophic failure. Besides, you can always bypass it by adding in alternative force multipliers. You don't need to go solely with how many bodies you have on the ground. Five Terminators surrounded by 30 Gaunts deserve to be insta-gibbed for their stupidity. Five Terminators in fortified cover, on the other hand, should probably be getting a combat resolution bonus to represent how much harder it is to shift a dug in opponent.