The trouble is that (a) the standard recommends but does not require that all binary-to-decimal and decimal-to-binary conversions be correctly rounded and (b) e

The trouble is that (a) the standard recommends but does not require that all binary-to-decimal and decimal-to-binary conversions be correctly rounded and (b) even if it did require correctly-rounded conversions, the precision needs to be absolutely enormous to guarantee that every double is exactly representable in the chosen format (which makes it wildly wasteful), otherwise you'd get different results depending on the rounding mode in effect at the time of conversion back to double.