When you PLAY the game the sounds work fine. When you watch FILM of the game often sounds are incomplete, inaccurate, too loud, too quiet, missing, etc.
Fact of the matter is it would sound like crap if you just recorded the film viewer and posted it. The sound track is an easier alternative to rebuilding 50 tracks of separate audio all perfectly queued and timed to the action on the screen. You basically do ADR for the entire video. A track of wind, which you bring up and down based on plane speed. A track for guns. Your guns. Their guns. Flack guns. Whatever. They all need their own. Then engines. Up/down with the throttle, but also doppler effect... Different sounds for engines in external vs internal.... Stall buzzer, flaps, gear, bombs, explosions, damage hits, losing parts, pilot wounds... These are just a few of the different sounds you would have to build in on your own. It's not easy.