The opportunity costs are just too attractive. The prime audience is an untapped generation of non-readers. In fact, the dominant audience is composed of voracious readers who seek out the net as a source of augmentation for other material already available to them in traditional form. Indeed there are many who surf to find glitz and jazz, but these people rarely return and very seldom are they the audience that advertisers seek. When it comes to closing a sale or developing a reading habit, however, the completion rate they generate is so low as to make them worthless readers, consuming more bandwidth than the revenue to be made off of them can justify.
Online readers demand rich audio-visual presentation.
In fact, they demand efficient presentation. Decorative sensory material may appeal once to a surfer, but the lifeblood of any site is its return visitors, and they are drawn by efficiency. This doesn't mean abandonment of all sensory material, but it does mean stressing the informational content of such material and making certain that it is an efficient use of bandwidth. Big advertisers and big sites, which have been at this long enough to have conducted their own research, are now talking more and more about limiting their graphics, not expanding them. We have to live with ads at the top of pages online.