#1: Because silence would mess up the scene. As an audience member, do you want to just sit there while someone's working on a computer? Click-click-click, letters silently appear on the screen. The audience needs something to keep it involved while the information is being retrieved and displayed. This would not be a productive silence such as in a dark scene filled with suspense and a killer lurking in the shadows.
#2: It's become convention, and convention is something that must be followed by filmmakers unless there's a very good reason to break it. Film audiences are genre and convention experts, whether they know it or not. They expect things to be done in certain ways. Creative convention-breaking can be very satisfying (e.g., Pulp Fiction), but if it's done in a mainstream movie and to a convention that's unimportant such as a computer making beeping noises, it could break the suspension of disbelief in the audience and pull the audience out of the movie momentarily. It could be done in a joking way by referring to the convention ("What the fuck is wrong with this computer? Why isn't it going beeping?") or in an art house film to demonstrate tedium or realism, but there has to be a good reason to do it.