Simply put: there is much more stylistic variation among older books than modern books. People who read mostly or exclusively modern books accustom themselves to a vary narrow range of stylistic and rhetorical devices, which they process easily and effortlessly because of their familiarity.
It would be hard for me to choose three authors more dissimilar in style (in the original language or in translation) than Marcus Aurelius, Plato, and Adam Smith. The fact that they all seem similar — because “very abstract” — is symptomatic of a very limited literary geography, beyond whose limits all is an undifferentiated other.
As for why they wrote that way: the idea that the quantity of “digestible” material per hour of reading is a useful or even a valid criterion of literary judgement would have struck most of the writers and readers of the past as bizarre. Efficiency was not something they looked for in their reading; nor was conciseness. On the contrary, a common word of praise, from Roman antiquity to the Renaissance to the Victorian period, was copious: a word which referred both to richness and variety of vocabulary (why say something once when you can say it three times in slightly different words?) and exhaustiveness of matter.
Edward Gibbon’s history of Rome, which far from a stuffy relic was a blockbuster hit in its day, sits on my shelf in seven volumes. Finlay’s history of Greece is in eight; the history of Greece by Grote is in twelve . Macaulay has six volumes of essays, not counting the four volumes of his history of England; and both were wildly popular. Could they have cut down the length? Absolutely. But why? People wanted seven volumes of Roman history and twelve volumes of Greek history and six volumes of essays. They liked reading it, so reading more of it was better than reading less. If you like chocolate cake, why have a single bite if you can have an entire slice?
It wasn’t simply that people liked longer books: they liked the leisurely way that the books told what they had to tell. They didn’t have a “quantity of information per hundred pages” ratio that they cared about. The reading itself was an aesthetic enjoyment, over which they expected, and wanted, to take their time — and in which they expected, and demanded, that the authors had taken their time in crafting a masterfully written, varied, and copious work of literature.
And this unfortunately is when we have to bring class into it. Because we should remember that the people reading these wildly popular blockbuster books of the past were… well, almost exclusively very well off. Mass education is a recent phenomonon. Mass availability of cheap books is a recent phenomenon. Mass free time for educated people to read books is a very recent phenomenon. What kind of person was taking their time over Gibbon’s seven volumes of Roman history (including the untranslated Greek and Latin and French and German and Italian in the footnotes)? People who had received a very elite education, probably didn’t have to work for a living (or not very hard), and who in any event certainly had servants to deal with the cooking and washing and childcare. Agatha Christie was a 20th century author, and as she said in her later years, she never thought she would be so poor that she couldn’t afford servants (and never rich enough to afford a car).
In other words, the people reading these books had leisure. And they chose to spend a very large portion of their leisure on books. And they had a lot of leisure to spend on these books. Why should they care about conciseness? They had time. Why should they care about a plain straightforward style? They were educated enough to follow an ornate style — and, again, they had time to read slowly and carefully, and savor the rhetoric of a Gibbon or a Macaulay. Reading, again, was itself supposed to be an aesthetic enjoyment.
Of course, you don’t have to be rich today to learn to savor twelve volumes of Grote’s Greek history. You can even do it without servants. You just have to have no social life.