Only bad or low quality cables would degrade performance. A good cable won't unless it breaks electrical specifications by being *too* long, but there are absolutely no problems with good quality cables of up to 3 meters.
The reason why most manufacturers add only very short cables to their products is much simpler: $$$. There is no valid technical excuse, no matter what you might have been told.
USB is fundamentally an electrical system that relies on voltage changes as its underlying mechanism. All cables, regardless of quality, have electrical properties such as resistance that are proportional to their length. Likewise, the signal degrades proportional to the length of the cable, so yes, the shorter the cable the better. Good quality cables help mitigate degradation and timing errors, but it still happens.