There are normally many subtle gotchas that affect numerical stability of a method, as well as great attention to detail required to handle degenerate edge cases. And that's before we get to subtle bugs that only trigger occasionally or are otherwise hard to detect.

It's not like NAG/ISML/HSL etc don't predate NR, in some cases by several decades. Libraries were always available, though back in the day you may have needed to pay for them.