Trees don't sink into the ground because (a) a tree's root system is as large in the ground as it stands out of the ground and (b) the pressure of the weight of the tree is distributed across the (large) surface area of that root system.

Trees aren't light, by any stretch of the imagination. That solid mass is mostly water. Try lifting up a 1' or 2' section of a tree trunk that's 1-2' in diameter sometime.