It depends on what kind of sites you tend to visit, I guess. I see this several times per day. But I often visit pages with math on them. Before mathjax became standard, inline images was the only way to do it. And visit any Wikipedia page with math: they use mathML and have fallbacks, but the end result is, you are looking at inline images. Also, I often see such things as sparklines and similar things, also inline images. The >99% may be true of what you consume, but without data I am skeptical about the numbers.

Also, some people use CMSs that substitute inline images for missing Unicode glyphs, and other things. If it’s done well, you may not even notice. You don’t know if the correct figure is 99% unless you have examined the source of a sample of sites.