Well if you look at the most recent open source supply chain attack on openssh, that used social engineering to add a backdoor to a project that openssh did not have a dependency on anywhere in it's SBOM. And with the xz example the backdoor had to be rushed out when it was deployed because the dynamic dependencey was being removed before the backdoor was completely in place. Doing a open source supply chain attack is not easy, fast or reliable for long.

It is not as simple as you say. Sometime it is better to know all of you dependencies are static linked at build time and specified when you are releasing your code. And the more sane you build system is the harder it is to add shellcode to your dependency's tarball and build scripts without turning peoples heads with random unsafe code.