If you mean by multiweapons more than one type of weapon, then think about how you should be able to fire them. Separately or at once? And how will you be able to control to fire with wich type?
Adding on to this... It would be too confusing, and there would have to be multiple crosshairs and zoom levels. Think about it- you'd need the railgun-loading bar on the outside of the crosshair circle, the plasma-launcher loading bar on the inside, and to top it all off SMG-crosshairs in the middle. How would you control which guns would fire? How would you do it quickly if you're in a life-and-death situation, like being shot at by an enemy at super-close range? You wouldn't have time to press a button or type in a command, you'd just have time to click at the enemy. And you can't exactly wait for railguns to reload in close combat at the spur of the moment when every second counts, can you.
There's your answer. Hope it helped.
You could use the numbers on top of the keyboard. But we're right. It's too OP, too confusing, and it wouldn't help with the keybindings, requiring neven more in depth stuff for the devs. There would also be a lot of keys to change from maybe fire all weapons to fire two to firing just one. The keyboard is already cluttered, and this is a fast paced game. There is no time to change options - you'll be snipped by then by a professional. It's just dose not work with this game and gfame type.