How do we know there isn't some predictable pattern from common urandom sources? What is meant to make the state non-predictable anyway? If I cared about it, so

How do we know there isn't some predictable pattern from common urandom sources? What is meant to make the state non-predictable anyway? If I cared about it, someone could just add a library on top of it that combines them with more entropy sources. Like camera noise, temperature changes, audio static etc. For some reason the author says not to build libraries on top of urandom because they might be hacked. Well if something has compromised your system anyway, why does that matter?