A lot of weight gain is water. Takes a lot longer to actually build keepable muscle. The body has to build capillaries and nerves to supply blood and activation to the new muscles respectively as well. Nutrition and training are the driving forces. What we are really aiming for is to establish a new set point. A set point that state where the body has recognized as what is natural to that body. If you have been say 170 lbs with x% body fat for 10 years, 8-12 weeks is not going to change that set point. Sure one may hit 200, but without maintaining long term consistency in nutrition and training, that person, without a doubt return to 170 pretty damn quick merely for that fact that the total metabolic, and regulatory profile of said individual has not changed much if at all. Think of it this way:
Igor is a pack rat, a horder. He has been so for most of his life. Igor wants a girlfriend but every time he brings a girl over she runs home. Girls just dont like such a messy house. Igor seeks the help of a therapist to help him with his compulsion to hoard things. A few weeks into therapy, igor cleans out his house of the crap. Igor is uncomfortable with the new change and tosses and turns every night in his sleep. He try's very hard to get a girl, but Igor already has a bad reputation in this small town. Finally, a new girl comes through town, and after a few drinks, he convinces her to come over to his house. Igor gets lucky that night, but this new girl is just passing through on her way to Timbuktu. Igor thinking all is cool and groovy now decides he would like to go his favorite place where he collected the widgets he loved so much. Only once, he says to himself. He gets a few lovely widgets and takes them home. The next day he gets more, and even more after that. Without even noticing his house is full of widgets, and he is once again sleeping well. He altogether stopped seeing the therapist and no longer gets lucky.
Our bodies our similar. If Igor had taken more time from hoarding widgets, Igor may have even scored himself a LONG TERM mate (or gains).
Changing life patterns and ones own biology takes time. We didn't become bipedal humans overnight, it took millions of years.