The fonts weren't as good, even the best that Addison-Wesley could come up with.
“I didn’t know what to do. I had spent 15 years writing those books, but if they were going to look awful I didn’t want to write any more.” (Digital Typography, p. 5)
Knuth started his TeX-and-METAFONT typography project in order to specify the appearance of his books; he wrote TeX so that he could use METAFONT. The former turns out to be useful even without the latter.
“I didn’t know what to do. I had spent 15 years writing those books, but if they were going to look awful I didn’t want to write any more.” (Digital Typography, p. 5)
(An example of their better-but-still-not-good-enough fonts, though I suspect that by then, after seeing the earlier worse fonts, he had already decided to solve the problem himself: https://tex.stackexchange.com/questions/367058/wheres-an-exa...)
Knuth started his TeX-and-METAFONT typography project in order to specify the appearance of his books; he wrote TeX so that he could use METAFONT. The former turns out to be useful even without the latter.