This has been a point of confusion for third party developers, particularly ones who were in the private beta (and saw versions of the sim that...cough cough...didn't work right).
Screen space ambient occlusion (SSAO)*, when enabled by the highest rendering settings, only affects exterior objects. This means scenery and aircraft-attached objects marked "exterior" for lighting.
I tried SSAO in the cockpit interior, and it had a few problems:
- The scale for occlusion in the interior and exterior of the cockpits is really different - I couldn't find one size for the effect that fit all cases.**
- When I went to apply the effect to our fleet, I found that virtually all of our aircraft already had occlusion baked into the cockpit by the artists. The dynamic AO thus provided almost no value and made the cockpit even darker than it already was.***
- SSAO only works at the highest rendering settings (and requires HDR to function at all), so if artists remove their baked AO, they're taking a pretty big visual loss in a bunch of settings.
So in net, it just wasn't worth going to dynamic AO in the cockpit. Our AO isn't as high quality as what you can bake in a 3-d program (given a few days of rendering time), and it's not always available.
The real win for SSAO is outside the aircraft, e.g. to cast AO around the wheels of the aircraft on the ground. That's an effect that you can't bake, and it helps a lot with lego brick scenery too.
* Nerd note: I'm pretty sure that what we do is actually HBAO
** We could, in theory, apply the effect twice with stenciling at two different scales.
*** The cockpit tends to be dark both due to errors in our approximation for indirect light (because most of the cockpit is in shadow, and thus only lit by indirect light) but also because cockpits are actually pretty dark compared to the great outdoors. But that's another post.