Part of this is a sort of history bias: it's a lot easier to talk about the battle of Cumae than it is to talk about That Time Those Guys Killed Some People And Took Off. And probably a historian bias, too. There's often a literal class war aspect when, say, your solidly upper middle class hoplites are being supported by a larger number of more irregular troops.

However, I would suggest that the reason is that it's effective. The development of it is discussed pretty deeply by Hammes Sling and the Stone, where he puts it in context of other developments in warfare, and "evolves," first in response to the previous solutions to the previous problems of war, then over time in each war. In the context it's shown up in the last century, the problem it's generally facing is how a much weaker conventional force can defeat a more powerful one, almost a response to the creation of the modern superpower, and one that's been pretty effective against it. Success begets repetition.

So while yes, you can find a lot of hit and run or other terror tactics throughout history (and maybe pre-history), it's an effective tactic against the sort of prior generation of war (according to Hames, more about the question of mobility and communication) and the world that form of war shaped.

Guerilla warfare has always been rather prevalent in the history of warfare, but has commonly been a sideshow rather than the point of focus. It isn't until the birth of the modern guerrilla in the Peninsula War that we begin to see elements of what would become a dominant trait in the guerrilla conflicts of the 20th century. While much attention has been put to this question as to why low-intensity conflicts has been on the rise since the end of WWII, the truth is that it genuinely hasn't been on the rise. If we look at the Cold War, we see as many conventional conflicts as unconventional ones. For every Battle of Algiers or anti-insurgent sweep, we have a battle of Cuito Cuanavale and Goose Green. The reason to why people might perceive it as being on the rise has more to do with the present day and the conflicts that the US has gotten itself involved with rather than with historical factors.

There is also the fact that NATO and the Warsaw Pact never went to war against each other. The fact that the Cold War was carried out without a major military confrontation between these two military alliances removes the stream of conventional warfare from Europe and puts it elsewhere. While European powers (or the Soviet Union and the US) never fought it out between each other on the plains of Europe, they did fight conventional wars on places that few knew or cared about. While the United States lost an unconventional conflict like the Vietnam War, they won a conventional conflict like the invasion of Grenada in 1983. The British managed to bring plenty of counterinsurgency campaigns to a successful close in Malaya and Oman, amongst others, but also fought a successful conventional war against Argentina in the Falklands 1982. While France proved itself equally unable to wage counterinsurgency in Algeria and Indochina like the United States, they did prove themselves in Kolwezi alongside the Belgium in 1978.

All in all, the notion that unconventional conflicts have become the norm in war is incorrect that is more influenced by contemporary conflicts and perspectives rather than historical fact.