For single-person households, cooking becomes even more expensive because you're buying these fairly large food packages and have to throw some of it away because it goes bad.
Only things I've ever had to throw out have been things that come in really large packages, like lettuce (extremely cheap and not the main part of a diet anyway, so it doesn't matter if half of it goes to waste) and other big leafy vegetables that are very cheap, like Kale or other non-essentials.
Most meats come in package sizes well suited for individual consumption:
1. Sirloin Steak = 2 portions
2. Package of turkey/chicken breasts = 2 portions
3. Rotisserie chicken = 2 meals/portions
4. Fish filet (salmon, tilapia, etc.) = various packaging, but easy enough to pick for 1 or 2 portions with no leftovers.
If you're a single person household and you buy a 20lb turkey for yourself...well that's on you for not being able to do basic arithmetic.
Thinking back to my bachelor pad days you are correct for processed stuff like making your own PB&J sandwich from bottled peanut butter and jam and purchased sliced bread. I'd end up throwing out moldy bread and jelly and rancid old peanut butter. Ironically to save money I had to cook healthier, stir fries of frozen veg and deli meat, salads I could eat in a day or two, fruits and veggies, stuff like that.
I could buy a hot dog from a hot dog stand or make my own at seemingly lower expense, but if I made my own hot dog there is no way I could gulp down a whole package of hot dogs and bag of buns at once, and of course the package sizes are different and the LCD is quite large, so I'd toss an opened package of hot dogs after some days of it looking or smelling questionable, or toss moldy hot dog buns, and swear off "cooking".
But boiling a wiener and putting it in a bun is not what most people mean by home cooking, nor assembling a PB and J sandwich.
Generally speaking you can save money by cooking at home, but not by assembling processed components at home. Or at least its much harder. I don't think I can run a net profit off homemade pizza if its made with pre-made store bought crusts.
If you pay a lot more as a single-person household, you have an excellent opportunity for improvement on your hands.
They can develop mold, which sometimes is not noticeable, but will attract booklice. I have had that happen to a bag of rice. There are also rice weevils.