This GitHub thread from 2015 was linked recently on another thread and it contains some interesting old staff discussion about the reasoning behind the current Casual/Needs ID/RG trichotomy.
At one point they did consider having separate “Casual” and captive/cultivated:
I think there’s likely some missing context from team in-person conversations after this, as the conversation continues on a tangential subject about RG and Needs ID being mutually exclusive before settling on this trichotomy without any further discussion of why the captive/cultivated category was eliminated.