Key:
>! It's in the vowels

Explanation:

>! Samsara words contain multiple sets of identical vowels which are repeated with the same spacing:
>! p**I**p**E**l**I**n**E** (IE-IE), **A**v**E**r**A**g**E** (AE-AE), sp**O**k**E**sw**O**m**E**n (OE-OE), **A**nt**E**j**U**r**A**m**E**nt**U**m (AEU-AEU), etc. 

Note:
>!As @el-guest pointed out, some of the non-samsaran words also fit this pattern (adequate, dilapidated), but they contain other vowels that are not repeated. Samsaran words contain exclusively repeated vowels.

Title:
>! Perhaps they are called Samsara words because Samsara is a repeated cycle of birth and death