Essentially, why is the pronoun the same in different uses for feminine but not masculine?
It is her car.
I know her.
It is his car.
I know him.
These pronouns follow the usual genitive and dative of Old English.
Masculine Genitive: his
Masculine Dative: him
Feminine Genitive: hire
Feminine Dative: hire
There can be no good explanation as to why these older forms were what they were.
The dialects that formed Old English are not attested before their arrival in England.
It is not generally thought that there is an accusative case in English presently. Object pronouns are used for both direct and indirect objects. Those pronouns are usually referred to as "objective case". Some grammarians might differ on that. I do not think using the terms accusative or dative for those pronouns is wrong, but it does not seem useful except, perhaps, in instructing English to persons whose native language has proprietary pronouns for those cases.