[magicalhippo]

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/

[2]: https://en.wikipedia.org/wiki/Imaginary_time#In_cosmology