There’s no such thing as a tree phylogenetically (2021)
Humans care about wood, so they made tree (thing which produces wood) a category, as it can be used to build bridges, ships and other structures.  Wood being dense also made it a better feedstock for charcoal (one can use grass, however).As far as why trees don't seem to evolve into grasses, (or grasses into trees), it might be all about C3 vs C4 carbon metabolism, the latter being a complex structural system that pre-concentrates atmospheric CO2 before feeding it into the photosynthetic biochemistry (which is the right way to do artificial photosynthesis as well).  This is apparently difficult to do in trees?https://pubmed.ncbi.nlm.nih.gov/32409834/> "Since C4 photosynthesis was first discovered >50 years ago, researchers have sought to understand how this complex trait evolved from the ancestral C3 photosynthetic machinery on >60 occasions. Despite its repeated emergence across the plant kingdom, C4 photosynthesis is notably rare in trees, with true C4 trees only existing in Euphorbia."And here we have the world's largest Euphorbia:https://www.wikidata.org/wiki/Q5851367