Even C these days, although they chose ' instead of _ :-(

My guess is that they did that because there are real human languages where ' is used as a thousands separator.

There are also real human languages where a space is used as a thousands separator, and an underscore is kind of the programming equivalent of a space.

A half space is British Standard for thousands separator if memory serves, also standard in Norway.

The reason they used quote is outlined in the proposal: https://open-std.org/JTC1/SC22/WG14/www/docs/n2626.pdf

TL;DR is that it's because C++ uses quote. The reason C++ uses quote (they considered underscore) is because of a very obscure feature of C++ called custom literal suffixes which I'd never even heard of, but numbers can be suffixed with a custom identifier, and since single underscore is a valid identifier you can't use that. (https://en.cppreference.com/w/cpp/language/user_literal)

Swiss German for example. But space, half space and dot is also common in Europe. Just not a comma, because this is used as a decimal separator (fractions) in most countries.