You've done this a hundred times. Blade against a curved surface you can't fully see, going by feel, hoping you catch the whole scalp without catching skin. A nick here, a red patch there, and now you're dabbing at it with a tissue before you can even get dressed.
If you shave your head every couple of days — like most guys who keep it bald do — that's not a one-time annoyance, it's a routine you're bracing for. Fifteen or twenty minutes with cream, a mirror angled just right, going slow around your ears and the back of your neck because that's where it always goes wrong.
So eventually, you start putting it off a day longer than you should, throw on a hat to hide the patchy regrowth, or just pay $20–$40 every couple of weeks to have someone else do it at the barbershop. Not because you don't care how it looks — but because your razor makes the routine feel like too much hassle.