It's just easier to advertise a single big game rather than several smaller ones. Even if you are interested in games it's impossible to keep track of everything that's being released. More casual players are aware of even fewer games. That's why AAA games still sell so well because they are the only games a lot of people are even aware of.
If the companies have to split their marketing budget between multiple titles, they would reach a much smaller audience. And even if one of the smaller titles would be a hit, it probably sells fewer copies for a lower price.