Because Codidact doesn't want to be a clone of SE but rather be its own thing, and that for good reasons.
Of course, every launched community over at Stack Exchange could be copied and transferred to here, even with all the questions and answers. The question, however, is: What is the purpose of that? Copying all the content doesn't bring users who participate and engage with that content.
Importing large amounts of content leads to the situation where a few users have to curate all the new stuff, a task which is infeasible. Furthermore, copying content doesn't really allow for an own dynamic to develop within that community as a very large part of the community, the content, is already a given fact rather than a to be dynamically generated and created part (on which communities users can leave their marks on). Think of it like that: Importing all the stuff means that basically ghost-writer authored every last bit of it and most of them are, just as the purpose of hiring a ghost-writer, not available after the fact, in this case having written a question and an answer. In fact, the content is rather "dead" in the sense that nearly every person who interacted with it in the past is not present here.
Copying is never a good thing to start with. Taking inspiration, yes, of course, no need to reinvent the wheel constantly but taking the exact same wheel and trying to fit it to a new car, well, yeah, not going to work most of the time. By allowing to create own communities here, with their own scope, with their own rules and especially with their own set of users paves the way for lasting and interested communities who like the Codidact idea and follow its principles.
In short, Codidact is its own thing (with its own communities and scopes), inspired by Stack Exchange for sure but not trying to imitate it. It has additional features, skips features or takes a different approach (for example voting and abilities).
Copying everything is just good for one thing in the end: for archiving. If Codidact's mission was to archive all the content from Stack Exchange, yes, that would be the way to go.