North American Black Walnut, when grown in it's preferred conditions, is anything but scraggly. Getting good, straight timber from it isn't hard, but it does take good soil, rainfall, and time - from nut to harvestable high quality timber takes something on the order of a human lifetime.
Walnut is basically a weed on my farm - I cut down 25 as saplings for every one I leave to grow to maturity - as it's preferred habitat is in colonizing grassy and weedy areas, where it's huge seed, and rapid growth as a young tree, allow it to get ahead of and above weeds, brush, and many other hardwood species.
Getting good straight timber from black walnut (or most walnuts) is absolutely doable (as evidenced by...us doing it for centuries!). At the same time it's more difficult than you're giving it credit for, though--"its preferred conditions" is a really load-bearing statement! It's not just soil and rainfall, though those of course matter a lot--as I understand it the biggest blocker to quality American walnut production is spacing. Those huge walnut trees have space to grow, and walnut trees grown under natural conditions tend to fight for sunlight. When they grow in natural distributions, they do tend to be thinner and have odd kinks in the trunk. Like you say, they're scrub trees. You can make them not be them, but it's work!
Contrast this to other hardwoods like maples or (most) oaks, which don't really care much about being relatively tightly packed. Silviculture is very much about strategizing how to grow trees in effective ways to get the outcomes you describe, and it's pretty fascinating stuff.
Spacing is indeed important, and if you are going to grow walnut on a plantation, you want to manage spacing over the first 20 years of the plantation pretty closely, with very dense competition in the early years to force straight upward growth, followed by thinning to allow space for crowns to gather enough light to maintain growth rates. You don't have to do that with most conifers, which tend to grow straight single boles regardless, but you do with most other North American hardwoods, including timber maples, and most species of oak.
Walnut is basically a weed on my farm - I cut down 25 as saplings for every one I leave to grow to maturity - as it's preferred habitat is in colonizing grassy and weedy areas, where it's huge seed, and rapid growth as a young tree, allow it to get ahead of and above weeds, brush, and many other hardwood species.