Why aren’t there any big chess trees?

Well, Chess Maps are 22 A0 pages, that is 22 sqm. You may call that big. Maybe you have a room of that size at home. That much area is necessary to allow for readably sized diagrams. Which are a key feature, as the goal of Chess Maps is to make all 3000 named opening variations visible. To keep the area as small as that there is no additional info included which would cover transpositions, alternative move orders, second names of variations or established yet nameless lines.