I think you're forgetting about practicality and not reinventing the wheel.
I'm going to use a car analogy here. Modern cars are incredibly complicated machines. They're also generally very reliable, thanks to about a century of development and engineering, and (relatively) inexpensive thanks to economies of scale.
If I want to transport myself to work on weekdays, and transport my girlfriend and maybe another friend on a weekend trip, and carry a bunch of groceries once a week, I can do all of that with a standard 5-person car. It isn't completely optimal for any of those tasks: it has more space than it needs for any of them, especially the weekday commuting. For my daily commute, I don't even reach highway speeds because I live close to work, so the engine is seriously overkill.
So I could have a custom-designed vehicle for each of these use-cases. Each vehicle would be a little less complex than my current car. But that's a lot to maintain, and would surely be far more expensive and less reliable, since each one is a one-off, requiring custom design and engineering, special parts, etc., and not benefiting from the economies of scale and engineering resources that a mass-market car gets. So instead, I just go buy a ready-made car and use it, and it works great and I'm happy.
Is PostgreSQL overkill for a lot of uses? Probably so. But it's designed to be used for all kinds of different tasks, and while it may not be quite as efficient for any of those tasks as some custom-designed solution, it's far more flexible, and it's readily available, plus it's benefitted from an enormous amount of engineering and debugging that a custom-designed solution would not. Things like CouchDB don't have nearly the number of users and amount of development, so while they may make sense for some tasks, the fact that PostgreSQL has more lines of code does not necessarily mean it's less reliable, in fact the opposite is likely true, just like an off-the-lot Honda or Toyota is likely much more reliable than some custom-designed car that someone built in their garage or some high-end limited-production exotic car like a Ferrari or Bentley.
The reason for using an off-the-shelf solution is because it's fast and easy and reliable. It doesn't matter if you're not making use of 80% of the features or capabilities. And software isn't like cars or engines; hard drive space is nearly free, and except for certain applications you're not likely to see a significant downside to just using a standard SQL database versus something more tailor-made. The main problem is cost (like with Oracle), but with PostgreSQL or MySQL this isn't an issue since they're free (and Free). It also helps that they use a standardized query language which makes them much more accessible.