Why must the velocity vector have a constant length?
But it's true.
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
edit: wait, if the Vector4.magnitude == 1, are dense objects actually just fast/hot?