Doombringer's link points out the common reasons. An airsoft gun that has the body of a sniper rifle, doesn't really make it shoot further or more accurately. Regardless of the body of the gun, you're still using air to fire a very light sphere, that has to move slower than a certain velocity in order to be "gamable".
If you're not going to spend more than $800 on a rifle, then generally speaking, if you can hit a guy with a "sniper" rifle, you can hit a guy with an AEG.
So because the sniper rifle doesn't necessarily perform any better than an AEG, and is often worse off, people recommend that you don't start off with a sniper rifle, because newbies are always disappointed when they realize that there's really no such thing as an effective designated sniper role in airsoft. At least not without putting a lot of money into a rifle, and being allowed to shoot with a hot gun.