Biology is complicated, ecology even more so...

An old forest is a result of multiple waves successions after disasters (fire, windstorms etc.), which are really hard to emulate. Some desirable seedlings are hard to grow artificially, others just won't prosper in situ unless/until very specific conditions are met...

After a long enough time, the forest will eventually revert to a fully natural state, but that time is way longer than human lifetime. It is a living organism of sorts and living organisms are much easier to kill than to re-create.