Question as answered: “Why do most female authors only write romantic/teen novels and tend to avoid heavy themes?”
Good grief, who or what have you been reading?
I must warn you, Your Question Is Naughty! You may want to don some kind of body armour for the other answers that are going to come your way.
I’m not entirely sure where you got this impression that female authors are not writing heavy books. They are and have been for a very long time. A small selection of great female authors:
Middlemarch – George Eliot (pen name for Mary Anne Evans). Religion, hypocrisy, marriage, the status of women, spousal abuse, narcissism, and political reform. Arguably the greatest novel in the English language, certainly top 10.
Beloved – Toni Morrison. Slavery, child murder, and horror. Won a Pulitzer, sold millions.
The Left Hand of Darkness – Ursula K. Le Guin. A keystone of science fiction.
The Handmaid's Tale – Margaret Atwood. Politics, religion, sex, and evil. Just one of her amazing novels.
The Lottery – Shirley Jackson. Society, politics, human nature. It’s terrifying.
The Grass Is Singing – Doris Lessing. Race, politics, segregation, colonialism.
To Kill a Mockingbird – Harper Lee. Race, justice, horror, child abuse, sex abuse, and coming of age. Won the Pulitzer.
The Age of Innocence – Edith Wharton. Marriage, sex, desire, society. Won the Pulitzer.
Harry Potter – J.K. Rowling. Love, death, maturity, power, and the nature of evil.
White Teeth – Zadie Smith. Politics, race, humanity. Won multiple prizes.
Once in a House on Fire – Andrea Ashworth. Child abuse, poverty, family, depression.
Wolf Hall – Hilary Mantel. History, politics, monarchy, sex, grief. Won the Man Booker.
We Need to Talk About Kevin – Lionel Shriver (woman). School massacres, grief, childhood trauma, family.
I could add a few dozen more here. I’m pausing for breath.
That doesn’t even touch the works of the Bronte sisters, Austen (not all fluff, in fact, not fluff at all), or, forgive me, the great poets like Sylvia Plath, Maya Angelou and Emily Dickinson.
To deal with your question in a kinder frame of mind:
There is a huge market for romance novels. Therefore, both women and men, who often take female pseudonyms, write romance novels.
There is a huge market for thriller and action novels. Therefore both men and women, who often take male or initial pseudonyms, write thriller and action novels.
Some of these deal with big and complex themes. Others deal with light fluff, like the prevention of the detonation of nuclear weapons by strong jawed heroes in faded blue jeans.
There is a smaller market for heavy themed novels, so it may appear that there are few authors writing that kind of book, but I don’t think you’d find much of a sex difference and I would argue that there are at least as many great female novelists as male novelists writing these heavy books. Or at least writing them well.
Many male novelists don’t touch on big or heavy themes. You may think they do because they put in some supposedly realistic, but actually laughable, politics in their action thrillers, but most novels are meant as entertainment and stay light.
However, you may assume female novelists are writing mostly lighter themes because their books are often colour coded pale cream, pink, or other light colours. The fact that the heroine was raped, or abandoned as a child, or has issues from other traumas, are covered under the fluffy nature of the marshmallow topping put over the underlying meat.
Most authors avoid very heavy themes because they want to sell books. Authors writing in female-oriented genres tend to make this more obvious because their books are marketed as lighthearted fluff. Authors writing in male oriented genres tend to hide this under supposedly important political analyses that are really excuses for them to play with bigger guns.
Do not confuse large weapons with heavy themes.