Most books written more than 100 years ago were poor by today's standards. Editors didn't really do a good job (did the job even exist?), and it shows. It would be like publishing someone on hackernews - there might be good insight, but the grammar, prose, and spelling leave a lot to be desired.
Except that of the books published 100 years ago, the ones that stood the test of time and are still published today are far more likely to be of high quality by today's standard, while most books published in any given year will inevitably be forgotten.
Did editors exist 100 years ago? Erm, yes they did? According to Merriam Webster, the term was used in its current acception in 1649. And the job done by editors could arguably have existed for a much longer time.
