why can't certain aircraft dump fuel?

All prototypes of airplanes are tested to land at their maximum takeoff weight, meaning without fuel dump - Is one requirements in test flights for transport category airplanes -

Boeing sells airplane types with option of having a dump system installed -

I have landed a few times over maximum landing weight in airline service despite the fact that I had a fuel jettison system installed in these airplanes - It was the decision of my operations office to "land as is" rather than waste expensive fuel -

Final authority to land overweight or delay landing is pilot-in-command's privilege -

I have never dumped fuel in 25,000 hours of flying - But "fuel jettison" is always part of recurrent training in simulators every 6 months -

You just try to make a smooth and gentle landing -

Don't make it a "Navy" arrival...!

After an overweight landing an "overweight landing inspection" is required - It takes one A&P technician 1 hour to do such, including sign off the paperwork in the airplane log -

For your information, in a standard 747, it takes some 35 minutes fuel jettison to reduce weight from maximum takeoff weight to maximum landing weight -

Simple, not all planes have that feature.

The plumbing and valves and controls for this feature add weight and cost to any aircraft. There is a magic formula that evaluates the weight differences between maximum take off weight and the absolute structural capacity of an aircraft to land, assuming, of course, the landing impact forces up to a determined level of energy. My last commercial plane was the Airbus A-320. The maximum allowed gross take off weight was not that high. It is a medium range aircraft with about a a six hour maximum flight time. It does not have the fuel dump capacity. Larger aircraft do. As to the need to land in an emergency condition, the damage to any part of the aircraft is minimized by the pilot. In case of a dire emergency, such as a fire or structural failure due to whatever reason, the landing possible structural damage potential is the least concern. The lives of the passengers and crew are of higher consideration. Dumping fuel is a specialized procedure since ejecting jet fuel into the ground can create severe hazards to people, animals and structure under the flight path of the aircraft. It is not taken lightly. Excellent question. Bye.