Recent dissection of the ethical concerns around AI mining of content. https://creativecommons.org/2021/03/04/should-cc-licensed-content-be-used-to-train-ai-it-depends/
If you really want to know: Have a look a this .. (attention math ahead ) https://mtrebi.github.io/2017/03/15/texture-mapping-affine-perspective.html ..or just simply: because they aren't rectangular ;)