How to create dynamic forms in Flask?

Question

Python

Please explain to the unreasonable why when creating dynamic forms it is necessary to use an internal subclass, why it is impossible to do setattr in the main class?
https://wtforms.readthedocs.io/en/stable/specific_...
The official documentation mentions https://groups.google.com/forum/#!topic/wtforms/cJ... but the link is not active, I didn't find any other ends.