Probably because the list is too long. Nearly all plants, trees included (with the exception of a few taprooted ones), will have a root system that extends no further than 18-24" into the soil with the bulk of the roots located just below the soil surface. Actual root penetration will depend on soil conditions (loose, sandy soil will tend to generate deeper and more far reaching roots; heavier or clay soils, less so).

I might ask why? Because if you are intending on gardening over a cement slab or somethingk, you will have other issues besides roots.
Linda C

Does this involve a buried pipe? ;o)

Al