Because old growth is also the most desirable wood and therefore valuable, and money is more important than literally anything else in the history of everything?
(I'm just spit-ballin' a guess here, as I don't really know the answer either…)
Fast growing trees tend to make inferior lumber. This is readily apparent with modern plantation grown species like fir and poplar that are not as high a quality as the same trees grown in natural forest conditions where their growth is slowed by competition for space.
>Fast-growing oak has widely spaced annular rings, sometimes up to 1/4″ per year (see fig. 3.3, above). This timber is exceedingly strong because it has fewer rings, which creates a great concentration of the dense latewood that grows in the summer. But the resulting timber is visually distracting. Its radial face comes out looking heavily striped. It can also be difficult to work; it has an
uneven texture resulting from the widely spaced transitions between the earlywood and latewood.
Faster growth is generally less dense growth as well. In the physical sense that is - tree rings are bigger, the material is lighter, and as you might guess - weaker.
Unscientific guess - trees are mostly carbon, so more CO2 can support more wood. Of course, we aren't simply adding CO2 while keeping everything else equal. Worse growing conditions due to drought or whatever don't help trees.
(I'm just spit-ballin' a guess here, as I don't really know the answer either…)