Q:

Why isn't coffee commonly sold in sachets, like tea bags? 

I was wondering why single serving coffee sachets never really took off the way tea bags have.  The only explanation I could find was that steeping coffee too long makes the coffee taste bad, but it seems like that could be avoided by removing the bag. 
Why would the end result be terribly different than a french press, Turkish brew, or cowboy coffee?  

A:

As a matter of fact, coffee is "commonly" sold in some kind of sachets. 
In Japan, there is what is called "one-drip bag coffee".  It consists of a flat sachet that you can unfold to deploy a small scaffold.  The scaffold adapts to any mug-like cups.  It unveils a filter containing ground coffee, as it unfolds.  You just need to pour hot water in the filter to get a decent cup of coffee. 
I will not link to any maker or brand, but most coffee dealers in Japan offer these bags---pretty convenient and offering a decent cup for the price.  These bags seem somewhat popular in Japan. 
I am not sure about global availability, though.  Several countries do not sell them (e.g. Europe, perhaps because of the French / Italian influences). 
 

A:

Because coffee is an agricultural/baked item and like bread, goes stale.   Between roasting, grinding packaging and shipping the bags, you'd ensure the coffee was never fresh and will never taste good.   The most you may get is a mass produced cup of horrible that might be passable or better than nothing to a small percentage of people.   
The other brew methods you list are generally used by people buying fresher, specialty coffee from shops.   Those methods are all a bit more intensive and tend to be used by people a bit more picky that folks buying Folgers. 

A:

The method of brewing you describe is commonly known in Poland, Belarus, etc, as 'Turkish coffee', althought it's something very different from the actual 'Turkish coffee', because coffee is poured over by hot water, like tea, and not boiled afterwards.  
In fact, many people use sprinkled tea instead of express tea (express tea, or sachet tea is usually of much poorer quality).  But many people dislike that method because of grounds that must be manually removed before washing the cup. 
The grounds of the coffee are very small grained, so they won't stop the sink, and I suppose it's the reason there's not so much demand on coffee sachets.  Another one is the popularity of coffee machines, with nothing similar in the tea world.  
The last reason that I see is, because coffee grounds are much smaller, it's technologically more challenging to prepare coffee sachets that would keep grounds inside.  So they would be (and from I've heard actually are) much more expensive, and not so many people are eager to pay extra for them.