There are manyreason I think gmod and minecraft are similar.
The first reason: The games can easily be modded
The second reason: The games are on a crutch. The only thing that is keeping this games alive are the modding community it has been shown countless times over and over again. That if your games support modding it will survive longer look at gmod and minecraft. Gmod being a game from 2004! But it still is thriving.
The Third reason: These games have had a long live but they continue to grow because of the people behind them.
The Fourth Reason: The developers of these games are slacking off and if these games were very hard to mod they would have had a short life span.