GAD and PTSD and all that spectrum are coping mechanisms, they're just maladaptive. CBT teaches you how to cope productively.

(personal opinion incoming), I think a person can get to the point where they need an SSRI as a stepping stone to even have the energy/capability to benefit from CBT, then they learn adaptive behavior, then they can dump the SSRI and let their new skills handle day-to-day life.

The "underlying reasons" are only useful inasmuch as you know what to look for so you can proactively start coping/remove yourself from maladaptive situations, but IMO most people try way too hard with the "why" stuff. The only thing that matters is "what now?"