I think there is nothing to argument to home training followers, who got to this site by aimed search of better exercises to train muscles at home. To all the others, who still cannot chose between gym and home training, I’ll try to answer the question in the title. And the answer is quite easy: because gym is a dependence. On trainers, on opinion of peoples around and even on weather conditions.
You come to gym to get your body strong, but you get to the dream shop with plenty of shiny metal machines. There are dozens of them, and every one is at your service. You get involved with process and forget about the purpose. You spend your time to learn how to use the machine, instead of training right. You wait for your turn on exercise machine, which will dictate your body how to move. Even if you avoid this machine madness, you will gather the inventory on all the gym territory at the beginning, which disappears somewhere at the end. Your training will turn to a quest, including weights searching and jumping from one machine to another. You will start to doubt in your abilities and the least progress.
And if someday, desperate, in far corner of your gym, you will decide to remember school PE lessons and just start to squat/push-up – HE appears. Personal coach, smarty-pants instructor. And suddenly it turns out, that you are squatting too deep, your elbows are too wide when pushing up, and at all, this muscle (here will be a long latin word) is worked better on that machine. And he, of course, will help you, tell you everything and show. Because he is the best coach, and also the walking instruction to all the gym equipment.
And again, you are pulled in this giant vortex of useless training. Where you spend your money to spend your time. Where the body is trained ineffectively and the brain degrades (most of exercise machines limit your moves so much, so that your brain has nothing to control). Where you are surrounded by advisors with smart look and the physique, worse than yours. Where talkers and rubberneckers sit (not exercise!) on the machines, that go to the gym only because it’s trendy or because the firm paid their year membership.
Do you need this? You have so much money and free time to spend on inconclusive training? You want to become stronger or learn how to use all the gym machines? If you choose strength (not its visibility) – train at home. Believe me, home exercises can make you stronger, much faster and efficient, than most of new-fashioned chrome mechanisms in gym. Home training – is the way to independence from everything, written above. This is the way of strong will and strong body, the way of constant discovery and improvement. The way, that thousands of athletes of past had been through. The way to victory.