As the sibling comment alludes to, it's otherwise they wouldn't behave according to what we measure. For a deeper "why", see Feynman[1]. edit: it's related to

As the sibling comment alludes to, it's otherwise they wouldn't behave according to what we measure. For a deeper "why", see Feynman[1]. edit: it's related to how spacetime with a finite speed of light is represented mathematically. There's some discussion here[2] which might shed some light. [1]: https://fs.blog/richard-feynman-on-why-questions/ https://fs.blog/richard-feynman-on-why-questions/ [2]: https://en.wikipedia.org/wiki/Imaginary_time#In_cosmology https://en.wikipedia.org/wiki/Imaginary_time#In_cosmology