You seem to be confusing "development" and "printing". Certainly, you normally print the negatives one at a time. This is about "souping" them.

Normally (for manual/home development) an entire roll of film is loaded into a spiral film holder, which is placed into the development tank (often more than one roll at a time) before any chemistry is added. The entire roll (or rolls) of film is treated as a unit during the development process—there may or may not be a water pre-soak (depending on the process), then the water is drained out and the developer added and agitated. When the development time is up, the developer is emptied out of the drum and stop bath added to halt development. It is only at this point that any images can be seen on the film without ruining the film immediately. The stop bath is then dumped, and the fixer added to remove any remaining unprecipitated silver and associated compounds, and after a sufficient period of time, that is dumped and the film is cleared (to remove the fixer), washed and dried.

With sheet film, you can handle each of the negatives individually during this process. You still can't see the latent images, but you know where they are: one image per sheet. They can be developed by hand in a shallow tray with a sort of slosh-and-shuffle technique, or they can be fixed to frames for a dip-and-dunk technique. Since each sheet is handled individually, you can develop each for a different length of time to develop the ideal contrast (according to the notes you remembered to write to yourself when you took the picture).

There are ways to separate negatives on a roll before development, but that almost always means losing one or more images. Basically, you have to cut the roll into several parts in perfect darkness, and while you can get pretty good at counting sprocket holes by feel, that doesn't make up for minor registration errors, differences from camera to camera, and how much leader you loaded before beginning to wind on in the camera. With 120/220 film, you don't even have the sprocket holes to go by.