Well most guys I see at the gym don't have massive muscles. Yeah you're right about the body weight exercises, but there's only so much you can do with those. They also don't provide you with a full range of motion, like some machines at the gym do. People go to the gym not only to gain mass and get bulky..or "massive". There's a crapload of aims. Some wanna lose loose fat around the muscle or just generally tone up. Like I said, body weight exercises are good and should be done regularly...but they're generally pretty limited. (they also get boring quickly)
I go everyday coz I have the time right now. I'm on holidays and I'm trying to take advantage of it coz once I start uni, I'm gonna be swamped. Also, on Sundays the gym is only open for like 4 hours, so I don't bother...I consider it my "resting day".