Most of the successful nations and empires in history were founded on lowland river deltas. What are the reasons for this?
The primary answer aside from the obviously challenging matter of large scale food production, sanitation, and resultant ease of population growth, is that economies and militaries rely on ease of transport and access.
Mountains are notoriously difficult to mould to human needs. A group might take many years to carve a single, narrow mountain pass to allow treacherous passage in single-file. To carve that wide enough to allow an army to pass safely - never mind all the mishaps that can happen during construction due to natural seams and fault-lines in the stone - requires modern technology. Never mind robust bridges across steep gorges etc. None of this was easily possible until the 19th c.
There is a reason that it is difficult for most of us to think of many mountain empires in history: there really weren't any (that lasted long in the face of competition from more conveniently placed kingdoms and empires).
Mountains are easy to defend from, it's true. But lacking a large population and the ability to make them mobile in short order, kind of limits your options in terms of taking over the rest of the world. This could be mitigated by moving your capital out of the mountains into a captured lowland city, but then... well... you're no longer primarily based in a mountainous region, are you? Remember that administration in former centuries relied very much on physical proximity, so again, having a mountain capital severely limits such administration.
It is conceivable in a fantasy world, that dwarves or trolls with great strength, great stoneworking skill, magical picks and countless existing tunnels (such as the network built by the Viet Cong, but in stone) might be able to forge an empire with its administrative centre in a mountain range.