Thing is, it still compares very favourably. Hell, going to school in 90s and 2000s, I remember old textbooks and exercise books books from 1950s-1970s, that my grandfather would occasionally give me. Those old English books (for English as second language - geographical context: Poland), in particular, were miles ahead of the stuff we had to buy for classes.

My working theory is this: those old books of my grandpa that I liked so much were, just like the manuals GP mentioned, written with reader's self-education in mind. They're supposed to teach you things as you read them and work through the exercises. In contrast, textbooks of my time seem to be written for supervised learning - the student gets content-light, fluff-heavy books that cannot be used in isolation, as crucial information is restricted only to teacher's companion books.

My cynical addendum is this: the latter is a great way for ensuring recurring revenue: the student book is not a complete solution, forcing dependence on teachers; schools are easier to convince that the last year's edition is obsolete and needs to be replaced with this year's edition, and then teachers will pressure students to buy new books, to avoid the hassle of supporting multiple editions of textbooks that are all incompatible with the most recent teacher's companion book. The market wasn't this diabolically wasteful a few decades ago.