It's really, really rare for a woman to be truly unable to breastfeed. A very few women simply never develop the duct tissue necessary to produce milk (I've heard this estimated at less than 1%, so it's highly unlikely). Prior breast surgery can affect a mother's ability to make milk (especially if duct tissue was removed, or the milk ducts by the nipple were severed; these are most likely to occur with breast reductions). Some health conditions (like thyroid disorders or polycystic ovarian syndrome) may make breastfeeding more challenging, but not impossible. Some mothers have health issues that necessitate taking medication which may make breastfeeding dangerous for the baby (although it's important to note that MOST medical conditions CAN be safely treated while breastfeeding). Or mom's illness may be transmittable through breastmilk (such as HIV, but even that's controversial). Some babies cannot suckle at the breast due to health or anatomy problems, but with proper support and equipment, Mom can provide milk for her baby either short- or long-term (and some of these babies can learn to breastfeed eventually).
The most likely reason a mother "cannot breastfeed" is because she received poor support and inaccurate information. Most women in the US experience a highly medicalized birth, which can mean baby is sleepy and not able to breastfeed well afterwards. Or mom and baby are separated. Many babies are given formula in the hospital, but most mothers don't receive breastfeeding support.
Most doctors and nurses receive no training on the normal course of breastfeeding and on providing breastfeeding support. Women are told they cannot breastfeed because their baby is allergic to their milk (babies can react to proteins that mom eats and pass into her milk, but NOT to mom's milk), because they're not making enough milk (in which case mom needs support and help to increase her milk supply), because their nipples are flat or inverted, because baby is too big/too small, because Grandma couldn't breastfeed. . . the list is endless. For a woman to find help with breastfeeding challenges, she has to be educated and dedicated enough to seek out help and accurate information on her own, which is challenging for a brand new mom.
We don't live in a breastfeeding culture; although most babies are breastfed in the first few days after birth now, by 6 months most babies have been weaned to formula. Most mothers don't have much, if any, support for breastfeeding. I think too many moms in our society think that encountering a breastfeeding challenge means she has to wean. I hope by the time my daughter has children, she is surrounded by other breastfeeding mothers, knows how to avoid common challenges, and has lots of resources for help if she does encounter problems!