The SRDs are intentionally incomplete.
Disclaimer: I am not a lawyer, and you should probably talk to a copyright lawyer before publishing derivative works for profit.
The point of the D&D SRDs as a document is to give potential paying customers a taste of the system, so they can see if they like it and potentially buy books. To quote the D&D 5th edition Basic Rules site:
We want to put D&D in as many hands as possible, and a free, digital file is the best way to do that.
They were also intended to give publishers the ability to easily write material using the core rules, to encourage more third-party publishing. It also exists to provide an easy rules reference (to a limited set of rules) for people who already own books, but don't have access to them at a particular moment. It is not intended to replace the books entirely, because Wizards of the Coast likes to make money, and seems to be of the opinion that releasing the entirety of their ruleset would cut into their profits.
Given this, it is likely that anything that exists in the books and not the SRDs is left out intentionally, so that players have a reason to buy the books, and so that Wizards can charge licensing fees for more of their work.
One other thing to note: There are two different D&D SRDs. Their legal provenance looks to be the same, but one applies to D&D 3.5 and one applies to D&D 5th ed. Material from one edition's SRD isn't compatible with the other's, though they both serve the same legal purpose.
To determine what is and isn't Open Game Content, look for the copyright notice at the front of the document or book. For a majority of D&D 3.5 books (possibly all, but I don't own them all), there is a note saying that they contain no OGL content. In the 5th edition Player's Handbook, there is no mention of the OGL at all, which means that nothing in that book is covered under the OGL. To my knowledge, the only 5th edition document covered by the OGL is the SRDs, which means that any content not in that document is not open content, and can't be used in a derivative work without the written permission of Wizards of the Coast.