[Tex/LaTex] Why does amsmath use fraktur for real and imaginary parts

Question

Edit: When writing this I was under the impression that this was amsmath's choice. It has now been bought to my attention that it is Knuth's.

When was it decided that amsmath would have the real part notation as an uppercase fraktur variation R rather than a Re? (same question for the imaginary parts).

What was the motive for this?

IMPO the bottom line looks better and more readable.

Also is it not the case that the latter are far more common? Why would amsmath want writers to keep redefining the latter? The \Re and \Im are convinient but return the former.

Answer 1

It does not answer the question why but my answer shows how to get what you want to get.

Option 1 (recommended)

\documentclass[preview,border=12pt,12pt]{standalone}% change it back to your own document class
\usepackage{physics}

\begin{document}
$\!
\begin{aligned}
z &= a + b i\\
\Re(z) &= a\\
\Im{z} &= b
\end{aligned}
$
\end{document}

Option 2

\documentclass[preview,border=12pt,12pt,varwidth]{standalone}
\usepackage{amsmath}
\begin{document}
If $z=a+bi$ then\\ $\operatorname{Re}(z)=a$ and $\operatorname{Im}(z)=b$.
\end{document}

Option 3

\documentclass[preview,border=12pt,12pt,varwidth]{standalone}
\usepackage{amsmath,amssymb}
\begin{document}
If $z=a+bi$ then\\ $\operatorname{\mathbb{R}e}\{z\}=a$ and $\operatorname{\mathbb{I}m}\{z\}=b$.
\end{document}