When I say she is not capable of taking care of herself - I mean that most people have some kind of initiative for existence and can get their basic needs met. W does not have this. W will claim to not eat all day, then come to me freaking out at 8pm because she is hungry. I just have never met anyone else who won't find a way to get food when hungry. But W will say "I couldn't decide what to eat" or "I have no food in my house".
Indeed, I've seen something similar with my disordered brother-in-law, and also with the pwBPD in my life--not only concerning food, but with other self-care routines that most of us take for granted. Things like cleaning the home, attending to one's health (medical / dental appointments), taking care of laundry/dressing appropriately, being responsible for bills, home mainetnance, etc.
I think Notwendy has a good point, that part of the issue is the emotional need to feel cared for. But I'd add that there are probably some deficits in executive function and general life skills as well. Part of the deficit is not having the scaffolding of a regular routine; in this case, we're dealing with the healthy routine of getting three square meals a day. Most parents teach their kids that they should have breakfast in the morning, lunch mid-day and dinner in the evening, and be sure to eat some fruits, vegetables and protein before any sweets. Right? Sticking to a normal routine means that people generally get the fuel and nutrition they need, while the schedule fits into normal patterns and societal expectations, such as school and the workplace. Most people come to realize that if they skip a meal, they get too hungry, so it's best to stick to the routine as much as possible. And they learn to read their body's physical cues, like, it must be dinnertime, because my stomach is grumbling.
What I've noticed with pwBPD and NPD is that routines tend to fall apart, because WANTS trump NEEDS. Reaching for junk food feels much easier and more gratifying in the moment than investing a little time and energy in preparing something nourishing. They just don't follow "normal" routines in my opinion. And that makes daily decision-making that much harder. Whereas I practically run on autopilot when it comes to making most of my meals, for them, they don't have an autopilot, except for the one that says, do whatever is easiest and most appealing moment to moment.
I think that the combination of a lack of a healthy routine, combined with long-term overfunctioning by the other adults in their life, results in the pwBPD not developing competence in some basic life skills. To illustrate, when I was growing up, I helped my parents with everything. I did all sorts of chores, year after year. One of my responsibilities was helping with meals. I accompanied my mom on hundreds of grocery shopping trips. I help pick out fruit, vegetables, meats, eggs, etc., especially items on sale. I helped with check-out and with carrying groceries into the car and house. I helped unpack the food, wash items that could be washed and put things away. The same went for making meals: I helped make breakfast and dinner the overwhelming majority of the time. By18, I had accumulated the experience of preparing thousands of meals, from shopping to prep to cooking to setting the table to washing up. Those thousands of instances of real-life experience meant that I was competent with the skills of feeding.
But when it comes to BPD, I think they miss out on formative experiences like that, because their emotional needs and instability are just too much to handle. In the moment, the adults around them determine that it's just too hard to ask them to contribute anything. Fast forward to adulthood, and oftentimes, pwBPD literally don't have practice with mundane skills of self-care, in this case feeding. Each individual step: planning, shopping, creating a menu, prepping, cooking and cleaning up feel too OVERWHELMING. Not only that, but the lack of practice means that whatever they might attempt to throw together just doesn't seem appealing. It's always easier to "delegate" and get someone else to do all this for them, or reach for junk food. As they do this, year after year, their executive function skills (and general life skills) appear to be childlike. Why? Because they are "stuck" at a child's level. Over-functioning by the adults around them, in the name of making life easier, means that the pwBPD doesn't get practice. Actually, that situation could lead to shame: the pwBPD knows that they're incompetent, and to avoid those feelings, it's easier to give up and not even try.
Of course, there's the sense of entitlement, too: since others have fed them in the past, they demand to be fed and cared for, as it's what they are used to and have come to expect.
Just my two cents.