Because vector graphics take up much less space. That's the joke.

Now I'm going to put the joke out of it's misery.

Most of the illustrations, formula, tables etc. in a math book could be vector graphics, most of them were in 90% of the upper level math text books I've ever had, usually in only 2 colors. Many math formulas can be represented and formatted directly using only Tex or LaTex. Mostly physics and math involving more than two dimensions would have more raster images, even color. But it's not like the publishers are going to be handing out PDFs with original vector graphics embedded. That would make high quality knockoffs trivial.