log2(10^16) = 53 bits of entropy or 300 years if your attacker can do a million guesses per second (the link says 1000 keys per second, but that's on the CPU).
You could also use `cat /usr/share/dict/words` instead of the `curl`, which is a much larger word list, but you get impractical passwords like "globular cellulose's malnutrition's dangling".
shuf is not a crypto tool, and the GNU coreutils are written to be cross-platform, even where /dev/urandom doesn't exist, or is unreliable. That's my guess, at least.
You could also use `cat /usr/share/dict/words` instead of the `curl`, which is a much larger word list, but you get impractical passwords like "globular cellulose's malnutrition's dangling".