Because the baseband is opaque to the software running elsewhere on the phone, proprietary, and not visible to inspection by security researchers.

Baseband processor has access to absolutely everything on all but Purism phones. Android is a guest OS.

I always thought baseband referred to the processor and firmware/software that responsible for the radio modem?

Do Android and baseband firmware run on one and the same processor(s)? Or are we calling ARM Secure World / TrustZone baseband these days?

Android runs on a processor that operates, or not, at the pleasure of the baseband processor. The latter has complete access to all memory, and complete control over the Android's CPU cores. Android and the cores it runs on have no access to the baseband processor's memory, or any control over its execution.