Because the pavement is harder than your head.

Overlapping wheels refers to riding with your front wheel beside someone's back wheel. If they suddenly decide to go left or right, they are going to chop your wheel and you are going to be the beginning of a pile up. Not cool.

The "buffer zone" is a strip of space between the white line of the traffic lane and the white line of the pedestrian lane. In theory, the space creates a safe zone between foot traffic and bikes and cars. Unfortunately, pedestrians tend to extend into it, defeating its' purpose and putting them in harms way.

Because it is illegal, dangerous, and makes you look crazy.

Yelling and shouting can startle riders into a panic. Inexperienced rider's first instinct upon panicking is often to grab a handful of brakes.

If the pack should roll up on a slow moving vehicle, the peloton must neutralize behind the vehicle. Swarming around a car is dangerous, illegal, and usually leads to the driver panicking and stopping. It makes us look out of control and just should never happen.

Riding beyond your ability only creates a disturbance in the flow of the pack. It is usually safest for inexperienced and/or unfit riders to hang out at the back of the pack. Riders at the front are comfortable riding very close to one another and are fit enough to drive the pace of the ride.

As mentioned in #5, startling other riders by yelling can be bad. If you need to call out something like "car up" or "right side", try to do it with a tone that won't cause people to panic. Yes, it is possible.