In HTML when used as images with the IMG tag, SVGs and bitmap images have a direct concept of ALT attribute for accessibility to screen reading. Font glyphs do not. You can add aria- tags around font glyphs but you can't add/set those in just pure CSS and so most icon font libraries don't bother. It also doesn't help that the most common tag for adding icons from icon fonts is the I tag (from ancient HTML for Italic, though EM is the preferred tag) which not only doesn't naturally support things like ALT tags it is even worse for accessibility by implying the wrong semantic meaning ("there are italics near here"? But there aren't).