You haven't really been paying attention if you think that's the case lol
It would not be the single biggest entertainment industry in the world by quite a fair margin if it primarily targeted a demographic that does not earn money. The bulk of the market is made up of people in their 20s and 30s, these are the people who were around and growing up during the days when video games actually targeted children. Infact I personally know a fair number of successful people, some who have earned enough to take an early retirement in their 30s, who spend plenty of time on such a juvenile activity while also having hobbies ranging from snowsports to skydiving. This is not 1990 nerd valley.
Adults playing video games are just wasting their time away, effectively sinking it into a black hole, you don't gain anything from it, no skills, etc to help you in life. If your an adult who has too much time to play video games then your life can't be very fulfilling. The gaming industry is typically greedy, always preying on those who think that by gaming it gives them a false sense of accomplishment all the while wasting more money as well. You don't learn anything by gaming, just staring at a TV and pressing buttons on a controller for hours, these are the types of people your defending, if I played video games as an adult I'd just be embarrassed! It makes sense for children to play games because they are still growing up and haven't lived in the real world yet and no wonder there is a huge stigma with adults playing games whereas with children it is more tolerated.
Remember this is a very unpopular opinion that people who self identify as "gamers" don't like to hear. Like I said in another post, adults who game should broaden their horizons, doing things that are more productive is a better way to fill your time.