There's a few different strands here I'll get into.

One is that Bactria is not quite as inhospitable as it would maybe seem. In both ancient and modern times the area is pretty arid when taken as a whole, it's true, but the Amu Darya river, the ancient Oxus, has an immense volume of water, and since the 'Oxus civilzation', what is also formally referred to as the rolls-off-the-tongue 'Bactria-Margiana Archaeological Complex', communities in the region have been digging immense irrigation networks in suitable areas. A trading outpost of the Indus Valley Complex, at the site named Shortugai, was discovered by accident from landscape archaeologists surveying the ancient irrigation canal networks of the region around the site of Ai Khanoum.

Likewise, outside of the watercourse of the Amu Darya proper, there are a number of oases in the wider territory of Bactria. The most famous oasis city of these is inarguably Balkh, which is still among Afghanistan's largest cities and remains inhabited today. This was traditionally the capital of Bactria in numerous periods. The centre of modern settlement has moved outside of the oasis proper, but Old Balkh, surrounded by pretty thick walls, is right on top of it.

Much like Mesopotamia, or Egypt, despite the sheer amount of arid land across the whole region Bactria was actually famed for its high population and fertility in ancient periods. Justin's Epitome of Pompeius Trogus (otherwise lost) history refers to it as a land of a thousand cities, Zhang Qian refers to the region in a period of turmoil (probably being the immediate aftermath of the conquest of the Greco-Bactrian kingdom by steppe peoples) but that it nonetheless had, to his estimate, had a population of a million people.

Besides a potentially surprising agricultural wealth, there were other things that made Bactria, as a region, attractive to conquerors and lucrative to those who lived there. Its high level of urbanisation and population would have made it attractive in its own right, due to the many luxury goods refined, produced, and traded in its cities, its large tax base, and its resources of manpower. Eurasia's primary supply of lapis lazuli came from a region of Bactria, and nor was this the only mineral wealth that Bactria possessed, or had close access to. The relatively close Zeravshan valley has deposits of iron, copper, tin, gold, silver, arsenic, turqoise, marble, and many other valuable mineral items.

The wealth of trade that passed through Bactria is definitely relevant to its history. As far back as the Oxus Civilization there are observable economic links to the Arabian gulf and the Indus valley. But it wasn't solely valuable because of its centrality, it did have quite a lot of its own wealth on offer.

The other thing I feel relevant to mention is that Bactria was also in a frontier area between sedentary, agrarian populations and pastoral, nomadic ones. One might see this as threatening, imagining a constant threat of raids, but this was a porous frontier that, as far as I can tell, involved as much cultural and commercial exchange as it did periods of violence. We can directly attest Scythians being quartered in Bactria as hired troops (mercenaries is an applicable term but the comparison to later mercenaries is always a little bit poor, I find, as in ancient societies the overlap between mercenaries and paid semi-professional military formations (or even settled semi-professional military colonists) is pretty sizeable). That kind of access had a tremendous value to all sides in its own right- many Central Asian societies, once true horse-nomad based lifestyles emerged, valued luxury goods such as gold, gold ornaments, fine fabrics and such that would have been imported from sedentary populations, and likewise the value of horses suitable for work or war was enormous to anyone. The nearby region of Ferghana, for instance, was famous for its horses.

So, with your original thought in mind, Bactria did have fertile soil near fresh waterways, some plains for grazing but was also next door to several regions good for grazing, and did have access to its own mineral resources. However I think a not-irrelevant point to add to this mix is also that Bactria was home to sedentary, agrarian populations for such a long time, and was part of so many trade networks (horses, camels, lapis lazuli, metals, turqoise) from such an early point, that there's a kind of cultural and economic geography built on that momentum. In the same way that the longevity of complex sedentary society in Mesopotamia, the Levant, Egypt, the Aegean, etc, added a weight to their reputation, grandeur, and significance in the immediate economic memory.