And they aren't better. Maybe some amateur New FD lenses have indeed lesser build quality than the corresponding 80's Nikkor lenses. For example the New FD 50/1.8 has really cheap build quality compared to a Nikkor 50/1.8 long nose. Or the nFD 135/3.5 to the equivalent 135/3.5 AI (I own the Nikon and Canon lenses I am mentioning, btw). But even then, the helicoids of the New FD are better (rarely show play and are smoother; don't require mainteinance as some AI lenses i've gotten did.)
Even then, i'd prefer the Canon New FD 135/3.5 over the Nikkor AI 135/3.5: The canon is smaller, lighter, and modern tests using a Sony A7 show that optically it's very good and the bokeh too. The Nikkor is also an excellent lens and one of my favorites.
Now, on the '60s lenses the Canon FL lenses build quality is slightly inferior to the nikkors until perhaps 1965 or so. For example the 85-300/5 Canon FL (which I owned) was fantastically built. (It was one of the most expensive FL lenses ever, on the other hand). Or my FL 19/3.5R, well built, nothing inferior in build quality (or optics!) to the Nikkor-UD 20/3.5.
The 70s FD line is well built, the good "pro-quality" lenses (i.e. the regular FD 55/1.2 or 50/1.4) have, for example, all diaphragm mechanisms running on a huge number of tiny ball bearings. Same for the "good" new FD lenses by the way. Only the cheapest new FD lenses got no such treatment: The nFD 28/2.8 lacks ball bearings, but the 28/2.0 did (as well as the older FD 28/2.8 and even the first FD 28, which is the f3.5).
Even then the "build quality" point is moot, because in the 60s and 70s, Pentax had the highest build quality in lenses by far. Change my mind!!