It's just that (a) you often won't notice those transient errors (one pixel in your multi-megapixel photo is one bit off) and (b) a lot of your RAM is probably unused.

Also, most modern processors use ECC for their caches (even when the main memory is non-ECC) and they serve the vast majority of memory requests, so it is unlikely that intermediate values in a tight computation are affected by non-ECC RAM. That adds to the "silentness" aspect of the bit flip in consumer systems.