My understanding is that the Android Open Source Project (AOSP) consists of the core operating system, a handful of commonly used apps, such as Contacts, Messages, Calculator, etc, and a suite of development tools.
Google also provides the drivers for the actual hardware used in the Pixel line of devices, and I believe it is the firmware for some of these hardware components that are only available as pre-compiled binary files (referred to as blobs) due to the proprietary nature of this industry.
When Google ceases to provide the firmware blobs for its hardware, it is difficult, if not impossible, for community developers to patch future firmware vulnerabilities as they become known.