Where I live (large American city) you can only find one brand (Folgers) of coffee sachets.
Maxwell House makes (or at least made) "Coffee Singles". If they aren't available in local stores, you could find them online. I've never had Folgers sachets, but I wouldn't expect Maxwell House Singles to be in a different league as far as taste.
Folgers isn’t exactly known for great coffee so I’m not surprised that their sachets don’t make good coffee. Would a better coffee make good sachet coffee?
Nothing precludes the possibility of making good coffee with a sachet. Start with good coffee, recently ground to an appropriate fineness, put an appropriate amount in a bag, let it steep in water of an appropriate temperature for an appropriate time, and you could produce a decent brew. But few, if any, of those conditions are met with the commercial sachets we're talking about.
Sachets pre-date pods. The first convenience coffee before pods were invented was instant coffee. It doesn't get more convenient than stirring a spoonful of coffee concentrate into hot water. Sachets followed as the convenience coffee that was supposed to taste more like fresh-brewed. "Taste like" are the operative words; it isn't really brewed coffee, it's essentially flavored instant coffee.
In a typical setting where commercial sachets would be used, the brew is poorly controlled. The water may not be at a good temperature for extraction (at least the Maxwell House Singles say to use boiling water, which is too hot), and the brew timing in relation to the actual temperature often isn't precise enough to avoid over- or under-extracting.
So they use a mix of instant and ground coffee, heavy on the instant. The instant provides the base coffee flavor, and the ground provides the "fresh-brewed" nuance. If the ground coffee is poorly extracted, you still get something that tastes different from instant, and the instant mellows any off-flavors.
You could make sachets and make decent coffee with them (easier with some supplemental paraphernalia). But even using just ground coffee, the results would be hit-or-miss if you merely plop it in recently-off-boiling water and wait a generic time (the typical conditions where sachets are your only option). With the commercial sachets, most of the flavor is instant coffee. Good brewing conditions make it a little better than the instant. Poor brewing conditions degrade the flavor of the instant (although it might still be better than 100% of the botched brew). The commercial sachets are never as good as brewed coffee because they contain little ground coffee, and what's there doesn't get brewed well.
I would imagine sachets are a much better option as they don’t require a dedicated machine.
True, they don't require a dedicated machine; just any source of near-boiling water. If you really crave coffee but there's none available, and you have access to hot water, that's when you will appreciate sachets.
Whereas pods are commonly available and many brands, flavors, variety exists. ... Is there some issue with sachets that leads to them being so unpopular?
The popularity of sachets and pods relates entirely to convenience. Commercial sachets let you make coffee when it otherwise would not be available. Most people are satisfied with the taste only as "emergency" coffee. The main issue with sachets is that they taste roughly as good as instant coffee, which is even more convenient than sachets.
Pods brew ground coffee under controlled conditions, producing a consistent result. The amount of coffee in an average pod will make roughly a 6 oz serving of "standard" strength coffee, so the brew will be weak or a little over-extracted if you stretch it to a larger serving.
It is extremely convenient in that you can choose from a variety of options, make a single serving of what you want when you want it, with a few quick steps, and cleanup consists of throwing the spent pod in the trash. You can produce better-tasting coffee by other methods, but they involve more time, effort, and cleanup. Many people find the pods good enough that they opt for the convenience and speed over better coffee.
Pods are becoming popular in office settings because it's a practical and convenient way to give people access to drinkable coffee when they want it, and it doesn't require making community coffee, or drinking or dealing with the dregs of an old batch. As people have become used to pods in various settings, it's also starting to become popular at home for people who want the convenience and speed. It does require a dedicated machine, but the machines have become widely accessible, and for home purchasers, it's viewed as just another one-time expense for a kitchen appliance, some version of which they normally own.
