People who don't want inbred, weak bits of genetic turd, that's who.
But the first few generations are typically great with good breeding practices. Problems arise when weak fish are bred to other weak fish and you end up with modern strains like today's guppy. Bent backs, wobbly posture and prone to illness if not pampered, they are a shame compared to their natural state.
Your rays look amazing. Nice patterns and everything. A good example of captive breeding for sure! But you couldn't have done that with weak siblings ten generations deep, now could you?