This isn't really true. Growing a forest is way more complicated than you might think - they don't just sprout spontaneously, as trees take a long time to grow and are easily kept down by fauna, landscape, nutrient levels, erosion, and many other factors.

I don't remember the details, but I believe it goes something like farm -> heath -> shrubland -> young forest -> mature forest, where each phase has a unique ecosystem of both plant species and animal life.

In an extremely heavily cultivated landscape like Denmark (seriously, look at a satellite photo), converting farmland back into forest is a multi-decade project requiring constant maintenance. Converting farmland into marshland (which is the "original" stone-age landscape in many areas) is a multi-century project.

Just like it was a multi-century project to convert it into farmland, by the way. Europe has been cultivated for millennia.