You make a good point. But many users aren't native English speakers, and in pretty much every other language, an 'o' is really pronounced ō. So a non-native speaker coming across the term will sound it out as no-stir or no-struh; or even no-stah in Japanese.

It may end up being nawster in America and no-stir everywhere else.

There's also a third path: nuh-ster — because the 'o' in "other" is pronounced 'uh' 😈

People make that argument in favor of a hard 'g' in "gif" even though a 'gi' is always pronounced as a 'j'.