Why are beings born?
There is no reason why "beings(sattānaṃ)" are born, there is causal for for it.  and causal for birth (jati) is Existence (bhava).  This is from dependent  origination or Transcendental Dependent Arising (pali - Pratītyasamutpāda).

Please make sure you understand the word "being" as per Buddha.  beings or sattānaṃ is a difficult term to understand.  ("being" is not the same as permanent soul).

recommendation for further study


Satta Sutta
Dependent origination