Why isn't no order (as opposed to ASC vs DESC) allowed for Mongo indexes?

Question

I assume other DBs behave the same way -

{
    Collection: db.Collection(VibeChatUser),
    Index: &mongo.IndexModel{
        Keys: bson.M{
            "Email": 1, // specify the field name and order (1 for ascending, -1 for descending)
        },
        Options: options.Index().SetUnique(true), // additional index options like uniqueness
    },
},

here I create an index on the email field - however it really doesn't matter what order they appear? Or maybe it does if binary search is used - not sure.

I am just surprised their isn't an index that allows for zero (unordered) - like so:

        Keys: bson.M{
            "Email": 0, // zero for unordered - this isn't real
        },

I don't know enough about indexes to understand this for sure - anyone know why an unordered index wouldn't be an option?