Why? Because it's the most efficient use of your time. As a beginner, you should spend the majority of your time doing skill work, which is where most of your endurance will develop. A beginner needs to become technically proficient before moving to advanced training. When you're spending time doing crazy cardio exercises you could be doing more skills training which will increase your endurance and your talent.
As far as becoming a stronger fighter, your time in the gym is best spent doing heavy sets of big compound movements. Since you're not worried about muscular endurance (that's done in skills work), you just want to get more powerful. Powerful strikes come from speed and your ability to produce force. Speed is taken care of by training proper technique. You ability to produce force is based on you limit strength in the necessary muscle groups. You get the most bang for you buck in training limit strength by doing strength training with barbell squats, barbell press, deadlift, and powerclean. Since 80% of your time should be dedicated to skills training, you only have so much time to spend in the gym. You want to optimize your time and cut the bullshit.
As a fighter, you want strong legs, hips, core, back and a strong shoulder girdle. The best way to get those strong in the least amount of time is by doing the big 6 compound movements in a rep/set scheme that allows you to progress in weight quickly and not spend hours in the gym. Really as a fighter, I would say 2 days in the gym for 1 hour or less each session is plenty of time for a beginner. A simple A/B split routine would be more than fine. For example.... One workout you could do squat/press. The other workout you do deadlift/powerclean. Each movement you work up to a single max set of 5 reps. Each week, you add weight to that max until you can't anymore. The volume is low enough to keep you good for skills training, and elicits enough of a training response to get stronger.
As your work capacity grows there's more you can add in, but that's just my personal opinion on a good overall plan for a beginner getting into fighting.