"I needed it," he says.

The exact nature of the trees is easier to see on closer inspection; branches and leaves weave together, creating walls and floors for a comfortable little residence, where the same flowers as fill the garden still grow freely. Nothing grand, but it's all that he needs or wants.

Some might need their towers, or even their libraries. But he's in the heart of what he is, here. Why overcomplicate things?
(Well... maybe he could make things a little bigger, nicer, more eyecatching. But that can come with time.)