When Below Zero came out it was clear that they had no idea what made Subnautica special, something that a sequel had no hope of replicating. Fear of the unknown and the drive to explore deeper. Once you've completed Subnautica you are a god of the sea, and that knowledge and experience cannot be lost.
Mobius Digital was able to make a successful DLC for Outer Wilds because they understood what made their game work, which is the player's own curiosity.
The rationale in the OP is that with archinstall, an inexperienced user will have no idea where to even begin diagnosing any issues post install. Whereas installing manually is sort of a barrier to entry that ensures you know what you're doing.