Why should I format a citation manually without bibliographic software?
All bibliographic software has a learning curve.  You have to invest time and energy up front to learn the system and set up your database, and only reap the benefits later in the decreased amortized cost of maintaining correct citations.  

If somebody doesn't think the tradeoff will be worth it (e.g., thinks they "aren't good with computers" or doesn't think they'll write many papers that the software would be useful for or is in a rush and isn't thinking about the long term), then they may manage a bibliography by hand.  

For example, I'm a devout LaTeX/BibTeX user and would never consider doing something by hand---except that I have done so on a few occasions when forced to use an incompatible format where it wasn't worth setting up a new toolchain (e.g., putting a few citations at the end of a PowerPoint deck).