computer people can't define them to mean something elseWell, the "computer people" did in fact do that, through natural language extension, which is often irregular and inconsistent.
It is this committee-designed "kibi", "mebi" and "gibi" which are prescriptive. And for that reason they are broadly rejected in the industry.
"Mega" is just a foreign prefix that can flexibly serve any purpose you want, just like a Greek letter in mathematics.
Just like the Greek λ can be "anonymous function", "eigenvalue", or "air/fuel ratio", the Latin "mega-" can be 1000000 or 1048576.
Nobody had any problem with this whatsoever, until storage manufacturers started lying in order to sell you a 781 Mb drive as 800 Mb.