Why don't light rays that stream through an ope window into a darkened room spread evenly through the entire room?

Answer 1

Why should they? As you said, the light "streams" or travels in straight lines, called rays, from the source, which is the Sun about 93,000,000 miles away, through your window to maybe a distance of twenty feet (if the darkened room is large). As the rays enter the window, the only thing that deviates their path is scattering off whatever dust particles and aerosols are in the air. This you can see as a "sun beam" in a darkened room, but only because the light has been scattered into your eyes by those minute particles. If the air is clear and dry, and you are standing to one side of the rays entering the window, you see... nothing! The light entering the window has to reflect or scatter off of something to enter your eyes.

On the other hand, try this on an overcast day where the light is coming into the window from all different angles. The light from the window WILL be spread (more or less) evenly through the room!

However, with direct sunlight, usually the light hits the floor, or a wall, and reflects from those surfaces in all directions (diffuse reflection), partially illuminating the entire room, but also absorbing some of the light. If everything is painted black, most of the light will be absorbed and the room will appear to remain dark. Some of the light rays may also hit a mirror (specular reflection), which re-directs the light rays to the floor, a wall, or the ceiling, again partially illuminating the room with diffuse reflection from those surfaces.

Light is invisible until it enters your eye and is perceived on your retina. Which is why you can't see the rays from a laser beam pointer unless there is dust, smoke, or fog in the air to scatter the rays into your eye. Except in movies and on TV where the "special effects" team makes sure you can "see" those invisible laser beams.