Because they're only alive and growing till they die and rot. Sometimes trees do both at the same time, rot from the inside out. You can't just plant trees for carbon capture, you also need to deal with how to permanently sequester the carbon.
Let's say that for millions of years a healthy biosphere grew around forests and the balance worked. Now you come to tell us it doesn't. Wouldn't you think it's a bit unconvincing?
I wonder why forestation is not present in this chart, as it is a low-cost carbon capture with side benefits. Sure, it is hard to scale, but reducing current deforestation rates would be a big step.