You only need to build the system and post outside the case if you have a small case or microatx or itx etc.... Where everything is crammed in tight.
In your case and in many gaming PC rigs, it's not necessary. People like myself purposely buy larger than necessary cases for futureproof expension and ease of accessibility and upgradability. If you install your components in a large case, all the components are easy to take out, the motherboard being the most time consuming but not difficult. Even then, takes me 5 minutes to take a motherboard out. Just unscrew your cards, unplug the wires, and take out the 6-8 screws holding it in, easy peasy.