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?
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).