It’s how the bibtex author field is defined. You don’t get free choice here. As far as I’m aware bibtex defines and as the separator
Yea but like... why? Typically you use human language operators to produce readable phrases, and this doesn't even approach readable english.
The person who designed it was solving primarily for lexical sorting of the author field, thought maybe having more than two authors was an edge case, and wanted the two author case to be a logical extension of the single author one?
The answer is probably who knows since we are talking about software from 1985 which was only updated once since 1988 to clarify its licensing