One of the main reasons is that people don't look out for them or suspect them to be in any 1001 other fields !. If you are driving along a country road, would you be concentrating on the road or would you be looking into fields !?.
When we used to look out for circle makers in the middle of the night, we found them on 3 occasions since 2010 and only because we set out on a mission to do so. Good quality night vision goggles are always useful - otherwise it becomes a Russian Roulette . Consider also that regular circle makers will probably have a pair of night vision goggles ( not always necessary ) so they to can scan the area for anyone lurking in the bushes - which is unlikely in the vastness of English countryside. You of course have to also guess which night they will choose to target a selection of thousands of hectares of land to use as their canvas, you could be looking in Wiltshire when they are all the way in Hampshire,etc. !
There is also always the misconception that there will be lots of people walking in the countryside in the middle of the night - hmmm you should first look at some towns and consider how quiet they are at night, so what makes you think the English countryside will be full of eager joggers, dog walkers and ramblers in the middle of the night ?. Ask yourselves the question - would you really go out into dark lanes of English countryside in the middle of the night ?. The answer is probably not !. ~
The reasons why circle makers are never caught probably exceed 101 reasons, so lets just concentrate on one for now.
Look at the image below - you can see the vast expanse of secluded land for a circle maker to choose. On this occasion a skilled circle maker has chosen elevated land, he knew perfectly well that no one was going to be around ( including the farmer ), especially in the middle of nowhere. His knowledge and confidence knowing they will not be seen can also give a circle maker an early start, probably just after sunset so they can mark out the design with circular markers and white tipped poles ( white tipped poles are more visible at night as well as white circular markers ), especially when the design is larger than normal. All skilled and regular circle makers will also prepare and plan their execution, its not always a random field selection.
So to catch a circle maker would require more than a a patrolling army - the landscape is just to vast to spot three or more guys crunching some wheat in the middle of nowhere !. You would need a force of helicopters with high tech night vision to scan hundreds of radial miles of countryside - ITS SIMPLY NOT POSSIBLE.
As you probably all heard by now, the only successful prosecution was due to the circle maker admitting to his crime - that's it that's the only one !. So next time before you ask the question why circle makers are never caught - think logically. The answers are all there, its the belief and influence by the money makers that creates the blindness to see clearly. THINK !
Even if a farmer suspects someone in their fields, very rarely will they venture in to check it out as its the human fear of not knowing what to expect. A plank over the head is not what a farmer would wish for !.