That’s like asking why I go to the cinema but enjoy eating potatoes. You are putting together two independent affirmations and supposing they are opposites.
CP forked from WordPress because the people behind CP wanted to stop floating on the same boat which is heading towards a different island, and wanted to stay on the island they already knew and wanted, called version 4.9. They want to make that island better and let people decide what plants to plant and what plants not to plant, via a democratic petitions system.
Your idea of a Multisite where plugins and themes can be user-independent is not impossible, but it does not seem to be in high-demand. You can always make your own copy of the boat and make it travel into whatever dreamland you like. It’s called the freedom of Open Source.