Why does Wiltshire host so many sacred objects in the spaces there?
Part of the answer is simply your/our perception that there are so many. Stonehenge and Avebury are massive and impressive. Over the last few decades, expressions such as "ritual landscape" have come into use. Prehistorians and the public link together many unrelated structures from a period of hundreds of years and think in terms of the whole area having been special to "the people" of the time. We can easily make the flawed assumptions that the area is "one thing" and the people of were a singe homogenous culture.
Stonehenge was built and developed over about 1,500 years — a period equivalent to the interval between the Romans relinquishing Britain, and the invention of the aeroplane. Think what 1,500 years has meant in historical times: Romans, Saxons, Vikings, Normans, 100 Years War, War of the Roses, Tudors, Elizabethans, colonisation of the new world, gun powder, the Civil War, the Napoleonic Wars, Victorians, invention of steam power, petrol power, powered flight. Now apply that insight to what societal changes may have happened in 1,500 years of prehistory.
Throughout those prehistoric times, people typically bred young and died young compared to people in a developed western society today. For simple arithmetic, if you allow 15 years per generation then Stonehenge was developed and used over a period of around 100 generations.
If we then lump together all of the nearby earthworks, burial mounds, cairns and platforms that were built, used, and abandoned over 100 generations, we can form a misleading impression that Wiltshire was somehow more special than perhaps it was to the people of the time.
This perception may be reinforced by the fact that remains of ritual monuments are more likely to survive in a visually impressive form than remains of utilitarian buildings like houses and barns.
Comparison 1. A quick Google search shows there are "over 40,000" places of worship in London. Tourists go to London and see Westminster Abbey and St Pauls Cathedral (compare with Stonehenge and Avebury) but they do not assume that the existence of 40,000 other places of worship means that London is an important religious centre of our culture, or a ritual landscape. The honour of being the oldest church in London is claimed by one which has a small part still standing from 675; the current St Pauls was finished in 1711. Just think what changes in personal belief, official dogma, official hierarchy and temporal authority — and of course, architectural style — happened in those 1,036 years.
Comparison 2. Wiltshire has an area of around 1,346 square miles, and most of the county is not littered with prehistoric structures.
Dartmoor has an area of only 368 square miles. Although less well known, it features Upper Erme stone row: the longest stone row in the world at 3,300 metres (around 2 miles). I used to spend a lot of time on Dartmoor, and there are small stone circles, rows, and individual standing stones all over the place. The Wikipedia page lists the 10 most impressive/famous, some of which are sites with several distinct monuments. In addition, there are 180 known cist burial sites, many of which were in cairn circles. There is also Grimspound, which is often described as a settlement. I have been there many times. It is not well positioned for defence, being overshadowed by a hill, yet it had a massive drystone wall and an impressive gatehouse. It seems obvious to me that Grimspound was important regionally and may have been some sort of administrative or religious centre rather than "just a settlement".
Comparisons 3 onwards! Although less visually impressive and less well known to the general public, there are important and substantial clusters of stone circles, henges, mounds, standing stones, stone rows, etc. in the Peak District (particularly Arbor Low), Oxfordshire, Cumbria, the Orkneys, Dorset, Wales, and many other places in the British Isles. The fact that they are not as widely known and appreciated as Stonehenge does not change the fact that they were clearly of enormous importance to the people who built them.
So, my answer to your question, <<Why does Wiltshire host so many sacred objects in the spaces there?>> is that, like very many other places in Britain,Wiltshire is home to a lot of things we could loosely categorise as "prehistoric ritual monuments". The big difference with Wiltshire is that, because of the fame of Stonehenge and Avebury, we are more aware of the proliferation of other sites nearby. We have then made an association between a range of sites that were built and used over a period of 100 generations, and probably many cultures and belief systems, and we have fooled ourselves into thinking the whole area had some special overarching significance.
It's a bit like ley lines. Find four ancient but unrelated things that happen to be aligned, and before you know it, we find ourselves looking for a fifth site on the same line, speculating about the reasons for the alignment, and ignoring the hundreds of sites that aren't aligned.