Q: Why can't any terms be added or subtracted from the License?
A: This clause ensures that each person that you distribute Open Game Content to will get exactly the same rights that you received when you got the Open Game Content yourself. Note that this clause means you can't restrict others from adapting your Open Game Content, or limit who can distribute Open Game Content, or add any other restrictive term. Likewise, you can't alter the terms of the license to remove sections that you might find objectionable, like the Product Identity definition.
Q: Does this mean that someone could take Open Game Content I wrote and distributed for free, and then put it in a product and sell that product to someone else?
A: Yes.
Q: To be clear: Does this mean that Wizards of the Coast could take Open Game Content I wrote and distributed for free, put it into a Dungeons & Dragons product and make money off it?
A: Yes.
Q: And they wouldn't have to ask my permission or pay me a royalty?
A: No, they would not.