Sassafras is an extremely hardy and a prolific grower. You will see a stand establish itself in a matter of 5 years to 15-20 foot plants. It also grows by root division, i.e., an established plant will send out new growth from older surface roots, giving rise to many new trees.
In many climates, it can be considered invasive, it's hard to keep the plant from taking over.
Most people couldn't tell an oak from a sassafras, and the ones that can generally don't know a damn thing about chemistry. A lot of farmers and old timers know the tree, but really, in this day and age, most people are clueless about anything that isn't setting between them and the idiot box.
If the concentration is around 6%, and the average yield is around 85%, then around 23 grams of safrole is in a pound of the root bark. That's hardly a shameful amount of safrole per pound, if you consider a few pounds can go in the average 2-5 liter distiller.
A mature tree can be taller than 50 feet, and 3 foot diameter, with roots as big around as your arm and bark on them up to an inch thick.