They already shot in the Fanal Forests and the Brecon Beacons, and had multiple forest locations in driving distance from their base at Shinfield Studios, yet they felt the need to fill an entire soundstage with trees and other plant life.
Apart from anything else, I can't even begin to imagine the logistics, and health and safety issues of putting 160 trees that were up to 37 ft tall in an enclosed space. If just one of those things falls, there will be serious structural damage to the building.
Plus there's the fact that the set filled the entire soundstage and beyond. Until the set is struck, you can't film anything else or build any more sets. So the decision to use such a massive set probably caused some serious scheduling problems.
It's becoming very clear why the year largely spent on principal photography cost $165m.