Because I don't have any. If I had one, I wouldn't have anything to put in it. I don't buy formula (that's one of the primary aims of nursing) and I don't have a pump. I don't have a pump because they are expensive and I have so little occasion to use it (that's one of the primary aims of staying home).
I don't get why people think pumping means Mom will get a break. It actually means the opposite. It means she will spend time hooked up to a machine getting schlooped like old Bossy before the time when the baby will need milk in her absence, and possibly again for her own comfort while she is absent from the baby. It means someone else will have to give the milk to the baby from a fake boob (more work for another person, although I know some mothers-in-law are pushy about wanting to do this work). It is totally inefficient. Pumping also doesn't work for everyone in a purely technical sense--saying, "Why doesn't she pump?" makes nearly as many assumptions about how a person's body works as, "Why doesn't she breastfeed?"
The number of stars in my eyes about breastfeeding is zero. I have had a plain rotten time of it. I do it because it is free and healthier for the baby and me. My commitment to it is strong, but purely utilitarian. I am sorry if this makes me a monster or something, but not sorry enough to feel even remotely like a monster.
Despite and because of this monstrous utilitarianism, I despise pumping. I have had to do it, what with the plain rotten times. While I prefer a little space when I nurse, I would prefer to be on another planet to pump. A pump makes me feel like some sort of inferior biological cog in a dystopian future; like my brain wasn't good enough to be connected to the Central Thought Reservoir so the alienbots settled for my lacto-glands instead. Nursing is meh. Pumping is mortifying. I could never pump without asking myself, “Is this really worth it?” When it was the only way the baby could get milk, it was worth it. If it was so I could leave the baby for a length of time so great that the baby would need a fake boob . . . not worth it. If someone walks in on a mom nursing, there's a very good chance he'll just think she's holding a sleeping baby, and she will almost certainly let him keep thinking that. If he walked in on her pumping, they would both die of embarrassment.
This means that occasionally I have to risk it. If the baby and I go somewhere, I can feed her before we leave and hope she doesn't want to nurse before we're home again. But it's complicated, because nursing doesn't always happen just because the baby is hungry. While we're out, I may need to find some way to keep her occupied and quiet because babies are squirmy and noisy. Nursing is the least obtrusive way for me to do this. It is immediate and quiet and still. I do not have to get up and walk out or shake a jingly toy or whisper-read a book. She is used to it and it is how she is most comfortable. Unfortunately, it is also nursing, which some people find inherently obtrusive. The best part is that you don't know who the obtruded upon people are until someone else tells you what the obtruded upon person said in some other time and place about how gross nursing is.
Why don't you just carry one of these around?
While we're on this exciting topic, the obtrusion problem is why the nursing cover is mostly a ruse. Those who wish to be covered are usually able to nurse in such a way as to not need a cover. Those who don't care that much may use a cover unsatisfactorily to the critics. The real trouble is that using a cover tips people off to what is going on, such that those who find nursing inherently obtrusive will feel obtruded upon even if they see no skin at all. They don't really want you covered. They don't want you nursing; at least, not where it might cause them to think about it in some non-abstract way, and especially if the baby is of a certain age (ie older than two days).
So, ye obtruded upon, sorry. Sorry these aren't good enough reasons for you. Sorry I don't carry a fake boob around because I know looking at fake boobs stuffed into babies' mouths doesn't gross you out at all. Try not to think about the fact that you don't know if it's nice, clean formula or gross boob milk that the baby is slurping out of the fake boob because that might gross you out if it were gross boob milk. May all the boobs in your life be fake.