Security today is very much about blind trust, certifications and not paying too much attention to the man behind the curtain. If your security software is open source anyone can point out its weaknesses.
Even then we still have the issue of binary blobs of firmware. Also hardware it's self may be exploitable. For a truly secure system everything would have to be audit-able. Also as soon as you run or insert anything into system you did not audit that whole system might as be treated as un-audited as well. Since what ever that was may have modified firmware or software.
That also exclude any subtle bugs that people may miss.
