Security experts have more faith in their ability to avoid triggering a scenario where a virus has the chance to gain a foothold. That's why there's such an emphasis on patches, to plug the holes they can't see to personally.

Not really. It's more of:

    - many would prefer to whitelist trusted software than blacklist malware
    - bypassing AV is trivial 
        - http://www.sevagas.com/IMG/pdf/BypassAVDynamics.pdf
        - https://github.com/Veil-Framework/Veil-Evasion

That many security experts use GNU/Linux or *BSD is definitely a factor, but many people still use Windows.

Great question, though. :)

I'm not a security expert, but I don't use any 'continuous scan' anti-virus on my Mac because I strongly suspect it would make the system more vulnerable, not less.

See, for example, https://lock.cmpxchg8b.com/sophailv2.pdf

In my personal and professional experience, A/V is effective but not in the way you might think. A/V has a very good detection and quarantine rate for situations where the infection vector is bloody obvious. When you open the .zip attachment to that email and then go right on ahead and run the .exe inside, that's when a good A/V product will save you.

What I'm saying is that I view A/V as protection from users, not from malicious actors. In a corporate environment with mixed-skill users, A/V is key. On my own devices, I don't frequently get into situations where A/V would be effective, the threats that are more likely to get me are more sophisticated.

This isn't to say that I don't have Windows Defender enabled, but I don't see a value return in purchasing a commercial product.

IME: No, it's because they know virus scanners are ineffective. Security experts on windows also don't run virus scanners.