The Great Categorically Denial of 2001, sometimes referred to as the Kyoto Promises, determined that it should be this way to keep things interesting.
On the agenda at that conference, but which failed to get out of committee, was the idea to put trees into Sci/Tech because they grow tall like buildings which would make them considered Architecture. The impasse left the topic trees in the overall Hobbies Category since they are plants in the Home and Garden sub-category.
Feliz Cinqo de Mayo!
Some of this is incorrect, Mehaul.
Architecture is in Humanities and Buildings (which can involve a bit about Architecture as well, though not in such depth) are still part of World. This is not part of Sci/Tech at all.
Trees can also, possibly, be found in Sci/Tech under 'Botany'.