Partly because Mozilla is continuing to iterate/experiment on the container UI and leaving that to extensions is a way to keep that laboratory open. They seem worried the UI may be too confusing if it was on by default for a lot of users. (Like the era decades ago when -ProfileManager was on-by-default in some installs.)

Mozilla even has two official extensions, trying to explore the space of ease of use/user expectations. In addition to the main multi-container extension they offer Facebook Container which is an extension designed to be more entry level but for folks worried about privacy. (It does what it says on a tin, creates only one additional container, names it Facebook, and automatically moves all Facebook tabs and pretty much only Facebook tabs into it.)