How come most of the time the female lead is an only child?

I really want to watch more kdramas where the female lead has siblings who appear from time to time.

I especially want kdramas where the female lead has an overprotective younger/older brother (My love from another star, Jealousy Inc.)

Kill me heal me
Stars Falling From the Sky
That Fool
Tatta Hitotsu no koi
Easy Fortune Happy Life
Golden Pouch
Mask
Bromance
Prince of Wolf
Entertainer
White Lies (watch the Korean version)
Lucky Romance
Song Huajai Nee Puea Tur
Temptation of Wife (2008) She has a protective older brother.
Hello Mr. Right (2016) She has an older brother who appears regularly.
High School King of Savvy (2014) She has a younger sister.
You're All Surrounded (2014) She has a younger brother but he doesn't appear often.
Shoppie King, Louis ; she has a little brother
Reply 1988 ; she has older sister and little bro
She Was Pretty ; she has a lttle sister
Uncontrollably Fond ; she has a little bro
Marry Him If You Dare
Bel Ami
Oh My Ghost
Six Flying Dragons
You're Beautiful

Just looking through my list, I realise how often female leads are only children or orphans...It's something hadn't noticed until you brought it up.  Thanks.
solomon's perjury (seoyeon has younger twin siblings)
boys over flowers (jandi has a younger brother)
coffee prince (eunchan has a younger sister)
49 days (jihyun and jimin are siblings)

Pretty much just taken off my lists. I tried to remove ones already listed, so apologies for any repeats.

Baby-faced Beauty
Brilliant legacy
Cheese in the Trap
Cinderella's Sister
Coffee Prince
Cunning Single Lady
Emergency Couple 
From 5 to 9 (Japanese)
Oh My Venus
Rooftop Prince (kind of a step-sibling...don't wanna spoil it)
She Was Pretty
Sungkyunkwan Scandal
The Good Wife (cameo appearance tho)
Hana Kimi/To the Beautiful You (cameo appearance)
You Who Came From the Stars
King's Daughter, Soo Baek Hyang
Cinderella and the Four Knights (step-sibling)
Fated to Love You
You're Beautiful  (I really shouldn't add this one, but it was last on my page and I wanted to feel a sense of completeness lol. The sibling is vital to the plot, but never actually shows up...)
Smile, You
Ex-Girlfriend Club
My Daughter Seo Young
Shopping King Louis
Wonderful Days
The Greatest Love
Trot Lovers
The Master's Sun
Wild Romance
Heard it Through the Grapevine
Goong
Five Children
Sunny Happiness
ToGetHer