Because golf is a game (not life) and it's based on stability and repetition. Do people spend countless hours practicing with specific equipment down to the weight just so they can change it later? It goes against the whole point of the game. Yeah change is inevitable but it's not something you should prefer. In golf.
No disc is made in a hermetically sealed laboratory to exact specifications. If they were designed and manufactured to such exactness they would cost way, way more than $20. We think a lot of our equipment but at the end of the day they're plastic frisbees. It's your job to know what they do, not the manufacturer's job to make you a disc to your exact specs. If you think you're entitled to that treatment, you're free to go on tour and out-duel Paul and Ricky and Paige to earn your own signature discs.
Even then, you will get x of the same mold and no two will fly exactly alike because this is disc golf, not formula 1.