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.)
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.)