Why can't I see the Pleiades straight on?

The centre of your eye has cells that detect bright light and colour. Around those are cells that detect dim light in black and white. What you describe is "averted vision" where you look somewhat to the side to place the image of the object in the region of these more sensitive cells.

Apparently your local level of light pollution is such that the Pleiades are not visible with direct vision.

Good observation!

This is a well known characteristic of the human eye called "averted vision." The part of the eye's retina which is most sensitive to faint light is not exactly at the centre of its field of view, but slightly off to one side. The cells right at the centre of the retina, called the fovea centralis, are optimized for resolution of fine detail. Astronomers quickly learn to look slightly away from the area where they are trying to see a faint object, and it often pops into view. Another trick from our days of hunting and gathering is that our eyes are extremely sensitive to movement, so tapping the telescope tube lightly to make it vibrate often brings out faint objects.