They aren't. A run-on, 70 word sentence for example wasn't easier to store in your head, or not require the occasional second read-through back then . Readers then still had to go through all of that. This "it's just outdated" idea is employed primarily to hand-wave poor literacy performance and reading requirements.

Gravity's Rainbow came out in 1973 and has a lot of the same difficulty, but people don't call it 'outdated' or treat it like a foreign language. These are just books you have to read slower than a news article, and that's alright, but there's a fine line between needing more time and not being able to get through it. That study showed it conclusively.

Because readers at the time didn't struggle with it as they do now.