Why does WP allow to view and media item as a page

It is desirable because you can modify that page, to introduce extra functionality.

e.g.:

• Comments
• Meta data
• custom fields
• titles
• descriptions
• Image EXIF
• showing geolocation data
• video/audio players
• thumbnails of other images attached to the post, e.g. the other pictures in the gallery

If it troubles you, you can ‘remove’ the feature, by making an attachment.php template, and putting a wp_redirect() to the images full size URL.

I would avoid this however, as this is a good opportunity to show off and make the most of your content, e.g.:

http://ma.tt/2011/09/wordcamp-bulgaria/mcm_3152/#image

Here, Matt Mullenwegs site uses attachment pages for gallery posts individual images, you can comment on images, you can navigate to the next image in the gallery and back, and upwards.