Women are underrepresented in a lot of roles throughout the world. It is usually due to some combination of social norms, expected gender roles, lack of opportunities, and old fashioned sexism.

It's also possible you simply don't see many female rappers because it's not the music you seek out. There are lots of very talented women in the rap scene. Though they certainly do not get the attention they deserve (see the paragraph above).

Edit: to answer the second part of your question, I'll go old school and say Rebirth of Slick by Diggable Planets. Missy Elliott and Lauren Hill are top-shelf, too. Other people here have mentioned Doechii; she's a new star on the rise.

I really love Doechii, especially "Denial is a river". But her whole most recent mixtape is amazing. Also do checkout her Tiny Desk performance, she is the one to watch I think.

I like dessa from doomtree, jean grae, doechii

If you're looking for recommendations, everything by Missie Elliot is worth listening to.

She might be mainstream/well-known enough that it kindof "goes without saying", but just in case you're not familiar with her.

I'll echo what others have said and mention Rainforest by Noname (or any of her other music)

Little Simz, Sampa the Great, and Snow Tha Product are some of my modern favorites

The majority of hip-hop I listen to is female. Tierra Whack (who imho is one of the world's all time greatest rapper), Latto, Glorilla, Megan Thee Stallion, Bea (despite the beef), Audrey Nuna, Young Miko, City Girls, Cardi B, Mariah the Scientist, Omeretta the Great, Doja, Ms. Nina, Renni Rucci, Lady Lashure, THOT Squad, Londynn B, LightSkinKeisha, REI AMI, Baby Tate, Tommy, Noname, Nitty Scott ... and so on

Not sure but there's also not a lot female metal musicians, but I have a feeling that's changing more and more. Prolly just because rock n roll used to be a manly guy thing? I can imagine similar things used to be like that in hiphop?

Gangsta Boo Princess Loko There were always good female rappers in Memphis

Doechii is the best female rapper releasing music right now, i won’t hear anything else

I’m not as into rap lately but I used to really like Noname

Keny Arkana - La Rage

YouTube recommended CupcakKe to me a while back and her style is fun. I don't remember which song I like best, but Old Town Hoe was a good one.

Here's a few I could suggest
Cap Chronicles - tg.blk
Love Being Used - tg.blk
Dora - Tierra Whack
Orion's Belt - Kitty ray
One life, might live - Little Simz

Angel Haze. She hasn't released an album in a decade but she still comes out with singles every now and then. My favourite songs are "No Limits" and "Weight"

I also love Meg the Stallion's Tramazine album.

Notable mention: "4AM in Toronto" by Miloh Smith & Junia-T

Like a lot of male dominant professions, women often have a hard time gaining success in this industry. Whether that's because fewer female rappers exist or labels are just less willing to sign them, I don't know.

I'm On - Kamaiyah

Doechii didn't win that Grammy out of nowhere, her new album is really good. My kids love Roman Holiday by Nikki Minaj, I am less convinced but they do love it.

Gangsta Boo

La Chat

Zigg Zagg - Suicidal, The Argument pt 2

B for Better - What's the Point?

missink is doing some awesome battle raps on PenGame