Having companions stick around has been something that RPGs have had as a staple since FF Tactics and Chrono Trigger. The End of Time in CT in particular is a prime example of how to let companions do their thing and you can take them whenever you want/need.
There is a camp mechanic already in the game. There is no reason to exclude companions.
Well ... there is a possible explanation: money and laziness.
Larian already has the DOS2 formula, and they are clearly reluctant to change it. Swen is adamant in this. It is cheaper in development to cut out companions, and cheaper in development not to try and change what they already have.
