Even video games with good writing are usually banal and puerile in their content. The exploration of themes in games is typically shallow and any didactic purpose the writers attempt to achieve is usually aimed very low. When an eleven year-old already inherently comprehends and accepts the lesson you are trying to impart, you know you're not dropping the bucket too deep into the well. A converse problem is that the themes being explored are so far outside of a player's daily concerns that they simply do not care.
A lot of game developers are really concerned about games not being taken "seriously". It's always been my opinion that if you have to ask for someone to take you seriously, you are not worthy of serious attention. If people find your content to be meritous, merit will be given. My concern about the lack of mature themes in games is personal. I think most games have uninteresting stories that explore irrelevant or trite subjects and they are really boring as a result. I don't care about pitting technology against nature; it's a trite theme. If it hadn't been explored in dozens of games already, it might be interesting. I don't care about focusing on high-level concepts like the "nature" of good and evil; it's far removed from anything I deal with on a daily basis and it is usually discussed in an explicit, heavily didactic manner.
Why doesn't anyone make a game about poverty? Why doesn't anyone make a game about capitalism and the rights of laborers under it? Why doesn't anyone make a game about racism? It's frustrating, because these are issues that are of direct, daily importance to a huge number of people. These subjects are either never broached or are explored through proxies that defuse the seriousness of what is being discussed. E.g. elves and dwarves might express shallow "fantasy" racism against each other, but you're probably never going to see two humans with different skin colors express racism toward each other in a serious exchange.
In rare cases, you might see the exploration of a subject like corporations vs. laborers or the religious vs. the non-religious, but the opposition is usually segregated into a "right" side and a "wrong" side. E.g. the religious turn out to be the bag guys, the laborers turn out to be the good guys. It's not an exploration of a theme as much as it is an exposition of the author's biases through various stand-'em-up-and-knock-'em-down characters. Exploration isn't really exploration when you're being led by the nose to a preselected destination.
I understand why game developers don't try to delve any deeper, though. Games are still considered escapist entertainment. While many media manage to have a wide spectrum of titles with varying themes and treatments, games are still very focused on pumping the player up and giving him or her a sense of tension followed by relief, accomplishment, and satisfaction. Things work out, the bad people get killed, and though one or two decent people might have been thrown into the grinder, it was all for a good cause -- and you know what that cause is.
People don't want to talk about things like poverty or racism or the pros and cons of a capitalist society -- because they suck. If these subjects had issues that were easy to solve, they wouldn't even be issues. They are problems that provoke dread, anxiety, confusion, anger, and a lot of other negative feelings. People don't want to escape to these things because then it's not an escape at all.
But they are real issues, and they are relevant. That's why they are serious, why they are topics of merit. I don't know if there are a enough people who are interested in playing games about such things to justify creating products to fill such a need. I have a low opinion of my fellow citizens of the world, so probably not.
But I really wish there were.