I think this is an “abstract algebra” v colloquial usage issue.
For non-mathematicians “x” concatenated with “x” gives “xx” and its natural to think of this as “two x’s”, hence + feels right.
In abstract algebra * is just an operation that satisfies different properties (I think “free group” and “word” are relevant terms). Since concatenation satisfies these properties it makes sense to use *.
Since Julia was designed with scientific computing (or numerical mathematics) in mind it makes a lot of sense they went with the mathematical version.