I agree with you, bonsai has too many limitations and constrains, which other art forms don't. In light of these limitations, a bonsai artist has an almost insurmountable challenge to somehow suggest a theme that has little to do with actual trees.
For instance, a story, such as The Family Tree, Tree of Life, Tree of Knowledge, or even Robin Hood and the Sherwood Forest (with its oaks) may be easier to "translate" into bonsai. Others may be almost impossible.
(Winnie the Pooh and the Hundred Acre Wood, on the other hand, can be a great inspiration

)