Why does the tree trunk grow in diameter even though you cut the roots and tips of branches?
Pruning does not kill the tree, at least if it's still pruning.
A living tree adds a layer of cells to its diameter each year. These growth rings are a defining characteristic of wood.
If you want the trunk not to get thicker, just kill the tree, that's the only way to stop the normal process of growth.
Root pruning is not a normal tree-care technique outside of perhaps bonsai.