New requirements, new features, old bugs being fixed, refactoring code to improve maintainability, writing tests for edge cases previously not discovered, adapting code for different kinds of deployment, ...
Depending on the workplace, refactoring or bug fixing is not something you just do. You have to create a ticket, meet with other members, discuss approach, scope, prioritise and only play when it is ready to pick up. The touching of the code is small fraction of that time.
Still, to write few hundred lines, doesn't take a whole week.
Many reasons to touch existing code.