Earlier quoted context omitted.
Because we could usefully distinguish billions of sites by their tiny little icons? ETA: To expand, I just opened the top 12 stories on HN right this moment in new tabs. I recognized the NYT logo in one favicon and the Apple logo in another. Of the other eight that did have favicons, those little icons told me absolutely nothing useful. A few of those sites did indicate their names in the titles, however.
I can distinguish the icons of every site I visit on a semi-regular basis.
Having only icons would be such an accessibility disaster.