Well actually they do. Ascending/rappelling equipment usually deployed on vertical cliffsor within caves can also be used on trees, but the commercial gear in the stores you mention is more suitable for that purpose.
I occasionally rig my stuff over a tree limb to check function and make adjustments, but not for recreation.
Hi @23485743 - Thanks for reaching out! While @hikermor is right that some of the equipment in our climbing department can work on trees and rocks, REI does not carry products specifically for tree climbing. We're always happy to hear about the gear our members need for their outdoor activities. Do you usually climb trees for recreation or for work? We’d love to hear more.
The book, On Rope, published by the National Speleological Society, 2nd edition, discusses rope equipment and techniques primarily for caving, but has a section devoted to other rope users, like arborists, river runners, fire fighting, and window cleaning. I believe there is a later edition, which probably has even more information. Highly recommended.