Well it is standardized , it is the metric system . Problem is after having voted for it in 1945 at the ICAO foundation , the US subsequently refused to apply and since the vast majority of the aircraft flying after WWII were theirs and in feet/NM , it continued .
From 'How Airliners Fly':
Curiously, for a technical industry, aviation has still not adopted standard units when quantifying parameters. Thus distances are measured in feet, metres, kilometres and nautical miles (and in some countries, statute miles). Speeds are in knots (nautical miles per hour) or metres per second (which is how some countries report wind speed). Masses are kilograms or pounds and air pressure hectopascals (millibars) or inches of mercury. The one exception to this pot-pourri of units is that temperature is universally recorded in degrees Celsius. Perhaps in the future aviation will switch to the exclusive adoption of metric measurement, which will obviate the need for personnel in the industry to make conversions – always a possible source of human error. Speeds would be kilometres per hour and vertical distance in metres. The current standard 1000 feet vertical separation between aircraft would change to the almost identical 300 metres.
The only amendments I would suggest are:
- retain altimeter scale in feet but refer to 'flight level' throughout (indication divided by 100)
- introduce GPS altimetry, so that setting errors are eliminated (with pressure altimeters as back-up)