What's wrong with https://en.m.wikipedia.org/wiki/Decimal64_floating-point_format https://en.m.wikipedia.org/wiki/Decimal64_floating-point_for... Or with C# 12

What's wrong with https://en.m.wikipedia.org/wiki/Decimal64_floating-point_format https://en.m.wikipedia.org/wiki/Decimal64_floating-point_for... Or with C# 128 bit implementation of decimal? Is there no good implementation anywhere?