Why does a pronoun as the predicate of an indirect object (e.g. "I gave her it") sound wrong?

Objects are usually nouns — be it direct or indirect and a sentence generally presupposes direct object if there is an indirect object. So the common realisation of the object through noun phrase is replaced by the use of pronouns. To function as an object is a typical function of pronoun in objective case.

They see me (here 'me' is DO)

He is giving me my book (me here is IDO).

She gave (the girl) (a doll)—

To save this basic pattern (SVOO; here OOs are all nouns) from degenerating in total confusion, one of the objects is kept (noun / pronoun - it is a sin qua non) as direct object suffixed to the verb and indirect is distanced from the verb by the supplement of "to/for according to demand of the verb but retaining objective character.

One problematic area of personal pronoun in English is that unlike Spanish it substitutes both living and non-living. The sentence becomes a riddle. Let us take an example.

I teach 'my cat' (IDO) 'how to chuckle' (DO).

By substituting both the objects with "IT" we get this nonsensical sentence.

I teach it it.

Hence grammar says direct object is must/ distance IDO if need be/retain one of the objects as noun if possible because substitution (pronoun) can not surpass the original (noun). By the way, this is my own explanation scholars may find fault with.

Robert Frost rightly remarked, "A sentence is a sound in itself on which other sounds called words are stung"

This is what's called a "constraint on" the rule (or construction or alternation) known as Dative (or Goal Advancement). Dative relates two different arrangements of direct and indirect objects.

• Mary told the secret to her mother ⤆ Dative ⤇ Mary told her mother the secret.
• Mary brought the book to Bill ⤆ Dative ⤇ Mary brought Bill the book.
• Mary bought the book for Bill ⤆ Dative ⤇ Mary bought Bill the book.

The constraint has to do with pronoun objects. There is an asymmetry:

• Mary brought the book to him ⤆ Dative ⤇ Mary brought him the book.
• Mary brought it to Bill, but not *Mary brought Bill it.

Stated one way, the constraint says that Dative does not apply if it leaves a pronoun direct object stranded at the end of the clause. Stated another way, it says that if a clause ends in two noun phrases, it should not be with a noun phrase immediately followed by a pronoun (the other way is OK, as long as the last one isn't a pronoun). Both seem to be true; and there may be other generalizations.

But nobody knows "Why". All we can do is report facts and patterns of facts.