How Come There's No Growth On These Ancient Stones?
You'd think that after a few years, never mind quite a few thousand years, that these supposedly ancient stones would be covered with lichen, moss, flowers, and weeds.
So it's no wonder that people ask why there is little or no growth of plants on the stones of Garn Goch, and tend to regard it as miraculous, spooky, or evidence of recency ('they can't have been there long').
It's none of those. It's simply that the soil on, in and around the stones is too acid, so the pH is too low for plant or lichen growth. Most plants thrive at a pH of 6 or 7, yet acidity close to mined and quarried stones can be as low as 2 when even the most acid loving plants like 4. 
This low pH is caused by a combination of three factors:
air pollution (which effects even lichens) 
acid rain. All rain is slightly acidic, and Wales famously has more than its fair share of it. Over time, rain makes all soils more acidic, hence the need for farmers to constantly top up pH levels with lime, which is highly alkaline
acidic groundwater: in this region 'many groundwaters are slightly acidic due to a lack of reactive material phases in the bedrock'.