Published flight manual limits are seldom where things go to worms, and generally where the manufacturer (and the FAA/EASA) have stopped testing because the number achieved meets the expected performance for the work the aircraft must do. Why is it forbidden to go beyond the values published? Because the FAA/EASA do not permit operations beyond that area proven to be safe and compliant.
This is true of speed, altitudes and most other published limits. Seldom does a published limit become that because above it things blow up. The one exception is probably CG, where the limiting value is sometimes that were control limits are reached somewhere in the envelope. But if the aircraft is somewhat over designed, and makes the planned CG handily, the manufacturer might just stop there, preen his feathers, and publish the limit.