Bhagavad Gita, Chapter 13, Verse 22

Purusha seated in Prakriti, experiences the Gunas born of Prakriti; the reason of his birth in good and evil wombs is his attachment to the Gunas.

Real attachment to enjoyment is named here as the actual cause behind repeated, real birth.

This isn't described as random or arbitrary, it's a direct, traceable chain, desire, attachment, then birth into a particular kind of body. Real cause and real consequence are laid out plainly.