Because if you put a villager on food, after several minutes, he's gathered maybe 200 food. If you have him chop wood for fishing boats, then once he's gathered 50 wood, a boat squirts out. Then when he's gathered 50 more wood another boat squirts out, and so forth. By the time he would have gathered 200 food, the fishing boats have gathered at least that much, and maybe more. Also, when you're going up an age, you can't produce villagers, but you can make boats. And even when you're not going up an age, you can produce fishing boats faster, because you can make more than one dock quite easily, while multiple town centers don't arrive till the Bronze Age.