Reasons Why Gym Workouts Beat Home Workouts

Attending the Gym Gets You Out of Your Comfort Zone.
The number one concern that you should have when you start to lift weights to build muscle mass, is to make sure you are always giving your muscles a shock to take your muscles out of their comfort zone, as that's the only way they will continue to grow. This means that you will need to focus your mindset on that.

The easiest way to begin adapting that mindset is to get yourself out of your comfort zone first. Taking yourself from one room to another in your home still means you will face distractions from activities, people or events that happen in your home. This will lead you to get side tracked at times. What usually happens then is that you are no longer 'in the zone' of your workout. Once that happens it's very hard to adapt that train insane mentality again. So the first step you need to take before taking your muscles out of their comfort zone, is to take yourself out first.
Which leads me to my next point.

Focus On One Thing Only.
You are more likely to succeed if you focus on one task at a time, instead of everything at the same time. Which is what will happen when you workout from home. Picture this...
You eat a good meal (No problem)
You relax and let that meal digest (No problem)
You prepare for your work (No problem)
You begin your workout
Possible problems
Phone rings
People within your household demanding your attention (Wife/ Husband/ Sister/ Brother/ Mother)
Those same people come to join and watch you workout
Your favorite TV program comes on
Your instant messenger starts ringing in the other room, which draws you away to have an online conversation (5 minutes will turn into 30 minutes)
You remember an important task that needs to be completed. You run off and do it.
This list could go on and on AND ON!
The point I'm making here is that there is no way you can continuously focus on taking your muscles to the next level, workout after workout, operating in an environment such as this. Actually, you can. But just know it will take you twice the time to get the same results, had you decided to go to the gym.
Which again, leads me to my next point.

The Gym Has The Tools For The Job.
Builders, Web developers, Fire men, Plumbers. It doesn't really matter which profession I talk about. If you take one of those professionals tools away, they will do a s****y job. And the same rules apply when the goal is to 'effectively' build muscle. Now let's put things into a visual perspective for one moment and make a hardcore comparison between the tools you have availble at home compared to the tools you have availble at the gym.
Home
Spare room/ Garage:
100kg Cast iron weights set, dumbbells, Barbell
A 'decent' weights bench
Chin up bar
Exercise ball, stability ball,Power blocks

The gym:
Locker room and shower (Hygiene)
Gym staff (Maintain the order of the gym, replacing weights, save you time from doing tasks that sap your energy)
Cafeteria (Relax before a workout)
Other gym goers (People to network with)

GYM BEATS HOME WORKOUTS!! i can go ALL day giving you reasons why!
Train like a beast always!