You are correct. They do this because corporations in the past have sued over even though rules, etc. are fair use. When they first started the OGL they gained a lot of goodwill from the community.

Short answer, no. There is a lot of nitpicky fine print and "nuance" involved but while you cannot copyright rolling a twenty sided die you can copyright a bunch of distinct and organized thoughts and specific groups thereof, such as the collection of rules that make up a class or subclass. If that class, subclass, spell, made up monster with a specific name and abilities, etc is published in some work that is sold for profit then legal action can occur.

Anything under creative commons effectively becomes public domain. If it appears in a WotC book, digital content, etc and is not specifically under CC, like say spells and subclasses from any supplement not included in that (such as Xanathar or Tasha), it is copyrighted and WotC can and will sue you if you republish it.