> Wikipedia is difficult to read due to the small font and large column width.
Narrow columns make text hard to read due to the eye being unable to see whole sentences at once. It's like the "Dick and Jane" books of old: "See Spot! See Spot Run! Run Spot Run!" Books for adults aren't written like that (as print novels attest) and there's a good reason.
What? That's the exact opposite effect that happens with wide columns. With a wide column, your eyes have to scan all the way left to right. With a narrow column, you don't have to scan as much.
Also the largest "adult" books (non-textbooks) that I have take up about 1/5 of my 43" 4k monitors screen. If you aren't adding a healthy text margin, my eyes have to scan left to right twice as much as with a book. If you want to talk about novels - my screen is 8 times as wide as them, so they are naturally using much narrower columns than most websites.
Books for adults aren't given narrow columns in the sense of a newspaper column, but they're absolutely typeset with attention paid to line length -- it's customary to aim for around 55–75 characters per line. On most displays, Wikipedia's lines are, by conventional typography standards, just too darn long.
Narrow columns make text hard to read due to the eye being unable to see whole sentences at once. It's like the "Dick and Jane" books of old: "See Spot! See Spot Run! Run Spot Run!" Books for adults aren't written like that (as print novels attest) and there's a good reason.