Because comics are written at least in the popular imagination for young men, who tend to go ga-ga over big tits and willing women. The idea that women might have friendships and companionships which might not have anything to do with sex is foreign to that mindset.

She-Hulk and Wasp are pretty tight knit, IIRC. Or at least they were until Disassembled.

a sismance? What the heck is the female equivalent of the word 'bromance', anyway?

Well, if we want to keep it thematically similar to the spirit and general usage of "bro" (that is, moronic college student slang), we could go with "homance." Which is either too stupid to work, or so stupid it works.

DC ought to include Phantom Girl and Shadow Lass from The Legion. Of course, Lightning Lass and Shrinking Violet were more than friends, even though nobody ever actually said so. Frackin' chickenshit editors... [mumble mumble]

Pre-retcon (and pre-Crisis) Black Canary and Hawkwoman (Shayera Hol)
Pre-crisis Supergirl and Batgirl (Barbara Gordon)
Lilith Clay and Wonder Girl (Donna Troy)
Pre-Zero Hour Phantom Girl and Shadow Lass

Yeah, the list is pretty thin.

And it occurs to me that Marvel has missed a number of opportunities with Aunt May, simply by neglecting to give her more female friends in her various incarnations. Yes, I get why the Lee-and-Ditko may was kept relatively isolated, to emphasize how vulnerable she was and how much Peter needed to worry about her, but Bendis-and-Bagley May? She was an active, spirited middle-aged woman, who's shown still working, and even SLEEPS WITH MILES WARREN ON THE FIRST DATE, so there's no reason she shouldn't have been given a group of female friends her age, even if they only got mentioned in passing or seen in glimpses. And once Ultimate May found out about Peter's secret, those friends would have become yet another group of people from whom the secret would need to be kept.

Dinah and Wonder Woman are pretty good friends too.

There's also Wonder Girl and Arrowette in Young Justice...

Cissie (Arrowette) and Cassie (Wondergirl) were pretty close back during Young Justice, no?

Empowered and Ninjette. (Seriously, the softcore bondage comic puts the interpersonal relationships in a lot of mainstream comics' to shame.)

Spider-Man Loves Mary Jane's Mary Jane and Liz Allen.

Runaways' Nico and Karolina, though that's an at least one-sided romance. And also from that series...Molly and Klara? Molly and Gert? Gert and Old Lace?

Oh, and on the villian side, we've got Poison Ivy and Harley Quinn.

Oh, and there's Catwoman and Holly Robinson!

See the real problem we're dealing with is "close." Female characters do occasionally run into other female characters and *gasp* form friendships with said characters, but then a new writer comes along and ignores that relationship. For example, Jimenez established a friendship between Wonder Woman and Lois, which a few other writers acknowledged--Rucka being one of them--but then trouble started. The Heinboot ignored any real possibility of Diana having any female friends other than Cassie and Donna--who are really more like sisters and therefore probably don't qualify for this discussion--and Gail's been working on establishing Wonder Woman's relationship with Black Canary and Etta Candy. And while I fully approve of Diana's friendships with both of those women, it basically means that the Wonder Woman/Lois Lane friendship is quickly becoming a thing of the past. At this point, they still qualify as friends, but not close ones.

Sue Storm and Alicia Masters?

Scandal Savage and Jeanette - they may or may not have a romantic history, but right now it's a very close and fond friendship - and created entirely by Gail Simone, so that's no surprise.

Outside the big Two and superhero comics, there's Maya and Alexandra in The Red Star; in fact all the friendships in that book are amazing and part of the reason I love it so.

Kate Spencer(Manhunter) and Cameron Chase and Agent 355/Dr. Mann from Y: The Last Man.

Sue Storm and Jennifer Walters

IIRC Volcana and Titania were friends before their transformation in Secret Wars

Kate Corrigan/Liz Sherman and Liz/Panya from B.P.R.D..

Peej and Terra were hanging out in Terra's mini and Terra showed up in the last Peej issue. They seem to be getting along okay, give them time and they might end up being BFFs.

Diana and Julia Kapatelis, and Etta Candy (Perez era)

Stargirl-Cyclone maybe? With Black Lightning's niece joining in also late in the JSA run.

I was going to say Storm & Kitty from X-Men.

Namora and Venus from Agents of Atlas.

Seconding Sue and Alicia, Dinah and Diana, and the girls from Runaways (although I haven't read much of them, I think that logically speaking they should be close).

What about Sue and Storm (lol...I mean Ororo)? Or did they not get to know one another since Storm and Black Panther joined the F4 when Sue and Reed took some time off?

Also, what about Charlie (Misfit) and Babs? Do they have a more "sisterly" relationship than just a Oracle-BoP agent friendship? Speaking of Babs, she knows a lot of women from when the BoP were around (and not just the "regular" members either - I keep thinking of that scan I saw where Dinah gathered together everyone Babs ever worked with to (IIRC) show Spymaster what she was up against if she tried to hurt Babs); she is probably friends with most (if not all) of them.

Babs and Cass?

Do Dinah and Sin count or is that a more mother-daughter relationship (not that those don't count, but...)?

Also, again although I've yet to read any of the actual stories, the Franco-Belgian comic series Yoko Tsuno not only has a female Asian protagonist, but apparently also shows her friendships with both males and females.

From the JLU and from the BOP: Dinah and Helena.

I would just like to point out the irony of using a Utena/Anthy icon in a post about platonic female friendships. I do agree that it's still somewhat ambiguous in the TV series, but...

Anyway. That reminds me I'd add Nico/ and Karolina to the list, provided they don't end up a canon ship.

Tends to run afoul of the tendency among fans to take platonic relationships between two attractive people (in any combination of genders) and make them non-platonic.