Why does Monte Carlo Tree Search reset Tree

Question

I had a small but potentially stupid question about Monte Carlo Tree Search. I understand most of it but have been looking at some implementations and noticed that after the MCT

Answer 1

Some implementations do indeed retain the information.

For example, the AlphaGo Zero paper says:

The search tree is reused at subsequent time-steps: the child node corresponding to the played action becomes the new root node; the subtree below this child is retained along with all its statistics, while the remainder of the tree is discarded