Taprooted trees are hard to propagate in a nursery setting. As soon as you dig them out, you disturb the tap root, which might stop it from growing, or worse, you might break the tap root altogether.
The ideal way to propagate taprooted trees is therefore to plant them where you want them and never move them again. However, it is not always possible or practical. For instance, nut trees are an easy prey to squirrels and chipmunks if not protected. When trying to propagate thousands of them, it is much easier to plant them in a nursery bed and protect them, which means you need to transplant them and run into the whole taproot issue.
Akiva Silver from Twisted Tree nursery made a short video showing his air pruning beds for growing taprooted trees. The idea is that the mesh at the bottom exposes the roots to air which stops it from growing.