It's a little more complex than that. You get branches (which manifest as knots in boards) where there's sunlight that can reach the leaves on the ends of those

It's a little more complex than that. You get branches (which manifest as knots in boards) where there's sunlight that can reach the leaves on the ends of those branches. Trees grown among other trees have to grow up to reach sunlight. The innermost part of the lower trunk might have knots from branches when the tree was young, but once they reach harvestable size, there's a fair amount of clear lumber down low. Trees grown in the open don't have the pressure to grow upwards, and end up with branches down low throughout their lives (from the center all the way through to the bark). As a result, there's very little clear lumber in them. Disclaimer, I'm a furnituremaker, not a forester. The above is somewhat better than pure speculation, but likely misses some important bits.