The short answer to your question is that ley lines are believed by some to be ancient because they were never simply "random lines on a map" drawn in 1921. Rather, Watkins claimed to have discovered evidence for a whole network of prehistoric pathways, and advanced sufficient evidence that he was correct to convince some of the readers of his works that they were indeed a system created by the people who built Stonehenge and other ancient monuments. On the other hand, the theory has always had some serious detractors, and I certainly wouldn't agree that the members of this group accept that "leys" are in any way an ancient concept.

In any case, there was always a bit more to the whole idea than this; the study of ley lines became popular in the late 1960s and 1970s at a time when there was a general surge of interest in "strange phenomena" of all sorts, and today they appeal to a particular group of "radical antiquarians" who in effect believe that they are rediscoverers and defenders of "the spirit of the British landscape" and who marry leys to a specific concept of Britishness (one that is very definitely not "Englishness" and has little to do with conventional notions of nationalism or traditionalism.) I wrote a little on the history of the subject a few years ago, and as this passage addresses your question, it may be of some interest to you. From Borderlands (1997):

In 1985, at the height of a violent series of confrontations between the police and groups of travellers, pagans and greens who wished to celebrate the summer solstice at Stonehenge, the British authorities announced that they were taking steps to prevent alternative festivities being organised elsewhere by stationing guards at other significant sites ‘where ley-lines intersect’.

The idea that, somewhere among their files on serial killers and terrorists, the police keep notes on the location of major leys may seem ludicrous, but if nothing else the story does demonstrate just how familiar the concept of ley lines has become since the idea was first proposed more than 70 years ago. Today the suggestion that straight lines linking ancient sites stretch across not just Britain but also other countries, from Europe to South America, is accepted by thousands of people who have never attempted to plot such an alignment on a map, much less actually gone out to ‘walk the ley’, as the serious researchers prefer to do. Many are also aware that leys are supposed to act as conduits for a mysterious sort of energy that is supposed to course through the earth like blood through the veins of a living animal, and perhaps even that if this energy is somehow blocked or stopped, a so-called ‘black ley’ is created, resulting in all sorts of psychic mischief and general misfortune for those living nearby.

That is the popular conception of leys, and all most people know of the broader subject of ‘earth mysteries’ – a discipline which actually deals with far more than mere ley-hunting, embracing geomancy (the art of controlling and refining man’s interaction with the landscape) dowsing, and the possible existence of ancient civilisations, and touches on archaeology and folklore. Yet the popular notion of leys is far from an accurate reflection either of the way in which such alignments were originally described or of the direction that earth mysteries research is taking today.

Ley-lines, as they were initially conceived by Alfred Watkins, the businessman and photographer who first described them, were no more than the remnants of prehistoric pathways: ‘the old straight track’ which guided travellers from one part of the country to another before the Romans came and built their roads, and helped them to progress in safety from one marker to the next. Watkins’s proposition – which he backed up with a considerable amount of fieldwork, mostly carried out in his native Herefordshire – was that ancient man deliberately placed standing stones, cairns, mounds and beacons to link outstanding natural features such as tumps or notches in hillsides and create the tracks. The implication, radical enough in the 1920s though much less startling today, was that prehistoric man was no painted savage, who built monuments such as Stonehenge with brute strength and little subtlety, but an able mathematician and a fine surveyor, much more civilised than many contemporary archaeologists believed.

Watkins went further than this, though. He suggested that, while the ancient trackways had at some point fallen into disrepair, their upkeep had at one time been one of the principle responsibilities of the local community, and that the later constructors of paved roads, castles and churches had taken advantage of this well-maintained network of sites when they came to build their own structures. As a direct result, he argued, it is still possible to follow many leys today, despite the havoc wrought on the countryside by the growth of towns, the construction of motorways, and modern intensive farming techniques.

In the seventy years since Watkins first published his theory, several thousand ley lines have been plotted in the English countryside. This is a superficially impressive achievement. Yet it is important to remember that there is no generally recognised definition of what a ley actually is. Many of the lines discovered by casual ley-hunters consist of only three or four points, whereas most specialists admit that such leys can easily be produced by chance alignments and agree that, to be valid, a ley should consist of a minimum of five points. There is even less agreement about how wide a line may be. Watkins thought that leys ought to be the width of an ancient trackway – no more than four yards. Modern ley-hunters seldom if ever work to such tolerances, and some have allowed their ‘alignments’ to be up to a hundred yards out. But there are real dangers in being so imprecise: the statistician and ley critic Robert Forrest has pointed out that the probability that ley-lines are no more than chance alignments increases dramatically if a single marker some 50 yards out of alignment is allowed to form part of a ley. Similarly, the length of a claimed line has a strong bearing on the likelihood that it is genuine: most ley-hunters accept that alignments of more than 12 miles are significantly more likely to be the product of chance. This does, of course, have significant implications for Watkins’s original theory of straight trackways, since such purely local leys would have been of little use in guiding a traveller. Towards the end of his life Watkins himself began to suspect that his leys might be more than simple pathways.

Years of statistical analyses have reduced the number of apparently genuine leys to a mere handful. One alignment which even Forrest agreed was beyond mere chance occurs in Yorkshire, where a group of giant monoliths known as the ‘Devil’s Arrows’–  the tallest standing stones in Britain, bar one, rising to more than 22 feet in height and spaced 200 yards apart  –is aligned with Nunwick Henge and the three Thornborough Henges, forming a ley 11 miles long. (Oddly, even ley hunters concede that the stones at the heart of this most famous of leys are not in a straight line. The northernmost of the three surviving Devil’s Arrows is noticeably out of alignment with its sisters and, as Devereux and Thomson, the authors of The Ley Hunter’s Companion, concede, ‘there is no doubt that they had a definite purpose... if the stones are not in a line, then they were constructed that way.’)  Hundreds of miles to the south, meanwhile, another well-surveyed ley has been identified running the 18 1/2 miles from Frankenbury Camp to a tumulus north of Stonehenge. As well as Stonehenge itself, this ley passes through Old Sarum, Salisbury Cathedral and an old earthworks known as Clearbury Ring. Again, years of careful work on the alignment of this ley has served to point up slight imperfections in the line: it passes just to the east of Salisbury Cathedral’s spire, slightly to one side of the central alignment of Stonehenge, and brushes the edges of Frankenbury Camp and Clearbury Ring. Critics may seize on this discovery, but perhaps it simply confirms that our distant ancestors, who marked out the alignment, were only human, and not guided by the superior technology of aliens or other superbeings, as another popular theory has it.

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