Environmentally because more diverse forrests tend to be more robust, depending on their climate (taiga is not very diverse). Tree growth doesn't slow down as trees get older. Their growth accelerates because: they have more foliage to gather sunlight and exchange gasses, everytime they add a growth ring it is a bigger ring than the last one, they grow taller or wider to get more sunlight and gas exchange. There is an optimal size/m² of course. Tree farms have ROIs and efficiency problems, they cannot take advantage of these qualities of forrests. They need straight timber and a payday at some point.

Ecologically because old growth forrests have a much more diverse system with big fungal networks, ecological nooks and crannies, etc. A system takes time to build.