Because they fit in them. Humans would like them too if they fit.
Boxes shield the back. So cats can focus on attackers from the front?
Cats like enclosed spaces. I suspect that evolutionarily speaking, it means predators can't sneak up on them.
Also, its just cosy as all get out.
Retrospring uses Markdown for formatting
*italic text*
for italic text
**bold text**
for bold text
[link](https://example.com)
for link