Because CXXOperatorCallExpr's argument 0 is the this-argument of the operator if the operator is a member. This doesn't correspond to operator declaration parameters.
Do not provide argument construction context for such arguments. For the remaining arguments, provide a context, even though argument index still doesn't match parameter index; the user would, unfortunately, need to work around that, as we can't satisfy both.
Actually supporting such this-argument construction contexts would address the FIXME that we've added in D32642 but this patch doesn't go that far.