Because there shouldn't be a way for scouts to kill people?

It takes practice to use one, and rewards headshots by dramatically increasing damage output. Besides, sidearms are SUPPOSED to quickly take down targets, and they are balanced by small clip sizes and low ammo pools (except for the SMG, which is just a really short range AR in disguise )

Think of it this way.

11 shots in clip at proto at 88 dmg a shot.

Damage in a clip? 888. Enough to kill a tanked med frame in a single clip with perfect accuracy. Once you start popping those headshots, you can deal 4-5x the damage, which allows you to realistically engage 2-3 enemies (assuming that you can pull off 2-3 headshots in CQC, which is a feat in and of itself)

Compare to the ScR.

79.2 dmg with 45 bullets in a clip. 3564 damage in a clip. Thats enough for 4 targets WITHOUT charging the gun and WITHOUT headshots. If you charge your shots and have good aim, that gun could rip though whole squads without having to reload.

This is what separates light weapons from sidearms. It's not the killing power, it's the ability to engage MULTIPLE OPPONENTS, for extended periods of time.

Scouts take advantage of sidearms because of the low fitting cost, and due to the fact that we don't need to be able to engage multiple targets for a long period of time. We want our kills to be done fast, and when you combine with a scouts speed, you can actually take advantage of the sidearms relatively quick reload times. Kill a target, get to cover, reload, and pop back out.

I also know for a fact that if you get too offensive with your sidearm, you run out of ammo. FAST. I only have about 3 clips to spare, so once you kill 2-3 guys, most scouts have to back off and reload. This is where our speed and stamina come in handy, it allows us to quickly flank, kill, and escape to reload and heal. It favors a fast based gameplay, and sidearms just happen to work well with it.