Because it is part of the theory of quantum mechanics itself. It is not just an empirical fact, it is a prediction of the theory itself. Similar behavior shows

Because it is part of the theory of quantum mechanics itself. It is not just an empirical fact, it is a prediction of the theory itself. Similar behavior shows up in Fourier Transforms [1]. When an audio sample is converted into frequency-space, it shows similar "narrowing/widening" relationship with the original audio sample. This again isn't a limitation of measurement, its just the relationship between the two bases [1]: https://en.wikipedia.org/wiki/Fourier_transform https://en.wikipedia.org/wiki/Fourier_transform