Why is it that in most cases when you apply an anti-reflective coating to a sapphire crystal, it’s only on the inside?

Anti-reflective coatings have obvious benefits when applied to a watch crystal, but they are sensitive and can scratch. If the coating scratches, it can look as if the sapphire is scratched or cracked, giving your watch early signs of ageing. We therefore only coat the inside of sapphire crystals to preserve the new look of your watch.