Not-invented-here syndrome in practice. Which kinda works for the n-th shitty web framework. But it goes really poorly in any complex domain.
It could be even the opposite: because many people can look into the code the code converges to the state of the art.
This results in new contributors being overwhelmed with the complexity of the classes and totally pushing aside the prior work in favor of starting at the basic level.
The module systems of common languages (like C# and Rust) tend to represent a "bag" of structs and traits, whereas a more advanced "evolutionary" language would represent source files or modules in a hierarchy of simple to advanced behavior.
Julia has many flaws, but they're a different kind of flaws. Mostly caused by choosing function overloads based on vibes, and reckoning with the pandemonium caused by modules being glued together in unanticipated ways.