Something about the wood feels unnatural and makes people want to speak in whispers.
Chapter one tells readers about the wood near the Foster home. The following is a short description from that chapter.
But the wood had a sleeping, otherworld appearance that made you want to speak in whispers. This, at least, is what the cows must have thought: "Let it keep its peace; we won't disturb it."
Notice the word "otherworld." That word causes readers to think "alien." I don't mean little green guys with antennae. By "alien" I mean strange, unnatural, and/or not of this planet. The wood has a special quality about it that, at the very least, causes the cows to stay away from it. Consequently, people stay away from the wood as well. The cows know something is strange about the wood; therefore, the cows travel around the wood. Their travelling eventually made a road. People ended up following the road too. Since there was no road through the wood, people didn't go into the wood, and nobody (except the Tucks) has discovered the magical spring of immortality.
In the end, however, it was the cows who were responsible for the wood's isolation, and the cows, through some wisdom they were not wise enough to know that they possessed, were very wise indeed. If they had made their road through the wood instead of around it, then the people would have followed the road. The people would have noticed the giant ash tree at the center of the wood, and then, in time, they'd have noticed the little spring bubbling up among its roots in spite of the pebbles piled there to conceal it.