Because it is more challenging than watching a movie. But it depends a lot on the game, why you play a specific game.
The essence is, we grew up with computer games available, so we play them. Adults do not stop wanting to play because they are adults. They just play different games. Or in the case of computer games, many of those are intended for adults, so no need to change the type of games.
Card games are popular among the older generations. And of course games for money, even dumb ones like slot machines. There is a reason why game shows are a thing. They are the let's play of the last century. You literally watch people play games, ranging from quiz games to dexterity based stuff. Or take sports. It is just games with more physical excercise. And of course, chess. All time favorite through the ages.
And just like video games, the reasons why someone plays chess, soccer, charade, poker, solitair, etc, vary with game and person. It is much easier to have a single player game with video games, though. But even there, people like to share experience and comment ;-)