Because a deliberate vulnerability is much easier to hide than actual malicious content.
One could probably sneak a buffer overflow or use-after-free into a C project they maintain without being noticed. Actually shipping a trojan is much harder, as observed with the xz-to-sshd backdoor.
Ah, so the next stage would have been to add a "bug" in xz that would trigger during the supposedly sandboxed execution, when presented with certain input files. Clever.
One could probably sneak a buffer overflow or use-after-free into a C project they maintain without being noticed. Actually shipping a trojan is much harder, as observed with the xz-to-sshd backdoor.