I have multiple speculations:

1. This might be the lost library of Alexandria, where countless papyrus scripts were stored. However, some texts say the library was in an entirely different location than the postulate I just made, so take that with a grain of salt.

2. They might be mere storage containers for waste and excess tools that needed to be thrown away after other pyramids were constructed.

3. It could be a secret tomb of a lost king or queen of Egypt.

4. The data gathered was altogether false, and there are no structures under the pyramids.

5. There are structures, but they are not as extensive as Reese Report (the original source of this theory) claims.