This is because gender in English is based on natural gender (i.e. maleness and femaleness) rather than grammar (i.e. morphology).
It is not so in many other languages, where the concept of grammatical gender is based on morphology and may apply not only to nouns and pronouns but also to other parts of speech such as adjectives and verbs.
In English, the idea is simple.
A male person or male animal belongs to one gender-class; a female person or female animal belongs to another. Simple, isn't it?
If English had grammatical gender then nouns, pronouns, even other parts of speech would have belonged to different gender groups depending upon their word-endings—and even these would have had exceptions!
Mercifully, English is much simpler than those languages.
Look at these sentences...
The pronoun he and the nouns lion, man, actor refer to male persons or animals. They belong to one class of gender. The pronoun she and the nouns lioness, woman, and actress refer to female persons or animals. Hence these belong to another class of gender.
Does this mean that English has only two gender-classes?
No. English has four.
This fact makes it easy for us to have clear divisions. The simplicity of this part of grammar in English comes from having four classes. I will explain this to you in a moment.
English divides nouns and pronouns into four genders in this way:
All males (and only males) are said to belong to the masculine gender. (examples: boy, man, landlord, god, tiger, horse, rooster, stag, he, etc)
All females (and only females) belong to this gender category. (examples: girl, woman, goddess, landlady, tigress, mare, hen, doe, hind, she, etc)
Nouns and pronouns that belong to this gender are either male or female, but we are not concerned about it. (examples: teacher, child, worker, baby, infant, human being, person, etc)
All nouns and pronouns to which maleness or femaleness doesn't apply belong to this gender category. (Material things: stone, table, gold, book; all abstract nouns: e.g. childhood, independence, intelligence, chairmanship, etc.)
Nowadays some words in the Masculine Gender are used as Common Gender. Everybody doesn't do it, but if you follow this trend, you will be considered modern!
Look at this example.
A teacher should not say lies. ________ should always speak the truth.
Would you put a he or a she in the blank space? English uses the pronoun 'he' for masculine, 'she' for feminine, and 'it' for neuter. These words are all singulars.
English has no pronoun to use for common gender, singular, and third person .
The nature of this problem and the various solutions offered, even strange ones such as the 'singular they' (the use of which has now become respectable) is another story!