The word "set" in my dictionary has a definition spanning an entire page. Most other pages have between 20 and 50 words on them. This implies that the word "set" will be chosen about 1 in 1000 times, giving only 10 bits of entropy, whereas choosing completely at random, each word would have about a 1 in 50,000 chance of being chosen, giving about 15 bits of entropy.

In practice, picking 5 random pages of a 1000 page dictionary, then picking your favorite word on each page would still give 50 bits of entropy, which beats the correcthorsebatterystaple standard, and probably a more memorable passphrase.