To keep it simple: you can only do so much in any given gym session. Not only would you be there all day, but you'd be doing so much work that the stress in your body ("every workout can add cumulative stress to your body") would run you down.
Obviously you could employ sessions where you do less work (exercises and sets) for each muscle group and get results from that. There are excellent "full body", "abbreviated" or even just simple A/B approaches. Most intermediate and more bodybuilding focused trainers though benefit from more split plans because they can make use of greater exercise selections and volume per training day.