Because you can send them text messages such as this one and they don't (outwardly) judge you...
Because they don't get mad at you when you recommend a gynecologist to them and then recommend their exercise class to said gynecologist. Hey, when girl parts are your job and you regularly see your GYN while you have clothes on, you forget that seeing people in public who have seen your parts might be awkward for other people.
Because you can regularly check with one another to make sure that none of you are crazy.
Because they don't care when you drop off the face of the planet for several weeks because you're too tired to talk to anyone.
Because they willingly have coffee with you every week while your boyfriend is out of the country for five weeks and your two other best friends are out of state indefinitely (which turned out to be temporary...thank goodness).
Because they don't get offeneded when you start referring to any combination of them as "we." As in, "we think it's time for you to kick jack-ass to the curb."
Because they don't get all worked up when you don't have enough cash to pay for your $3 drink and they pay for it for you because they know you'll get their $3 drink next time (speaking of which, B I think I still owe you $10 from the time you owed me $40 but only had a 50 dollar bill to pay me with and I didn't have change...or did I?)
Because without them you wouldn't be able to find a house to live in, you'd get fat from eating poorly and not having any exercise classes to attend, you would know nothing about car insurance, your dog would be without her dog BFF, and you wouldn't have anyone to have coffee or dinner with on weekends you were home alone.
Added 6/4: Because they let you live in their house with their husband while they are halfway across the country for 6 months.