The
JAA Manual of Civil Aviation Medicine gives some explanation of why there are limits. It's all to do with the optical effects induced by correcting lenses.
Chapter 13, section 4.3 "Spectacle correction of ametropia" states:
One of the reasons for setting an upper limit is the optical aberrations caused by correcting lenses. These optical errors increase with increasing lens power and towards the edge of the lens. With modern materials used in high-quality correcting glasses problems are most unlikely to occur inside the range of ±5·0 dioptres.
Distortion of the image due to peripheral angular magnification narrows the effective visual field.
The prismatic deviation gives rise to double vision in myopes and a ring scotoma in hyperopes.
In anisometropia, the refractive state is different in the two eyes. When corrected with glasses, these give a dissimilar magnification – a condition known as aniseikonia. The illusion created is particularly disturbing during the initial stages of wearing anisometropic spectacles; it is better tolerated when the glasses are prescribed at a young age. As a general rule, an anisometropia of 3 dioptres can be tolerated; if problems arise, a special evaluation as to the practical applicability is necessary.
Actually I think the UK CAA is keen to get the "irrelevant" limits abolished. But bear in mind that there are 32 other member states of the JAA that get to vote on the standards, and not all of them agree.