In other words - pre-European explorers - was actually climbing the mountains a thing? What would they be looking for up there? Or is the climbing-oriented culture a more recent invention?
This isn't necessarily about the Sherpas themselves, but their ancestors. Archaeological and genetic examinations of the famous rock-hewn cliff-tombs of northern Nepal -- most of which are impossible to reach without climbing equipment -- were made by Eastern Eurasian people between the second and first millennium BCE, nearly 2-3,000 years ago. [1] The 'Eastern Eurasian' genetic affiliation is significant, because unlike the Western Eurasian peoples more common in Central Asia and northwest India, the peoples of the Tibetan plateau and northern Himalayas -- including Sherpas and many Nepalis -- have considerable genetic admixture with Denisovans (a species of archaic human that, like Neanderthals, reproduced with early humans), which is why Tibetans are very well-adapted to high-altitudes. [2]
This means that the rock-cut tombs of Nepal were being carved out by people who were intimately familiar with climbing in high places and transporting things like corpses and burial items up cliffsides, so their orientation towards climbing was definitely a pre-European phenomenon [3]. As for why they would bury their dead hundreds of feet in the air, it's not known for sure, but it was most likely religious in nature: it's believed that the Buddhist and Zoroastrian tradition of sky burial was influenced by the practices of these proto-Tibetic people.