1. Most sniper rifle designs are based on, or converted from assault rifles, and can be made to shoot just as fast if you wanted to. Even the Barrett M107 is a semi-auto.
2. The AA-12, USAS and after-market Saiga shotguns can hold up to 20-32 rounds and pummel an area on full auto, emptying their mags in 4-5 seconds flat. Load grenade rounds and there'll be nothing left.
3. Pistols and submachine guns can both feature full/burst fire, are far easier to conceal, and their small size allows them to be effectively used indoors, which is how most wars are now fought - on a close-quarters basis, rather than an open field.
4. Even heavy machine guns can only be fired in short 3-second bursts before they start to overheat. While Rambo may have taught you otherwise, an overheated barrel, besides being a dangerous thing in it's own right, may start firing off shots on it's own and furthering it's demise; this is called a "runaway gun". As a result, machine guns must absolutely be used in pairs at the very least, so that they can take turns laying down the lead.
5. Even if a weapon can fire tons of shots really fast, they are almost never used that way. It's a waste of ammo, does nothing for your ability to hit the target, and simply pisses everyone off.