One huge review on tree rooting depths found that while 95% of all roots are found within the top 3.6 feet of soil, some trees can push down to an average maximum depth of about 23 feet to tap into those deep water reserves.

https://richmondtree.com/how-deep-are-tree-roots/

Tree roots rarely go more than like 8 feet down, its expensive to pull up water from that deep. Also NA forests require regular burns, many species seeds won't germinate unless there's a fire and the only way they compete with faster growing pines is being safe from regular, low intensity burns.