Why does D language do not support BigDecimal type?

[BoQsc]

There is Money datatype that can be provided by using a third 
party package: https://code.dlang.org/packages/money

But that's only for money, what about math?
Why such fundamental as BigDecimal is still not included into the 
D language itself?
There is BigInt.

If it is unavoidable to use Floating point, how can I quickly and 
simply understand the rules of using float to make the least 
error, or should I just find a third party package for that as 
well?


There is an article on that, but it is not that straight forward:
https://dlang.org/articles/d-floating-point.html

Basically any thing that I find on Google, that include 
explaining floating point are badly written and hard to 
understand for the outsider lacking ability to understand 
advanced concepts.