I guess we all have some gaps in our knowledge and I wouldn't be bothered if someone didn't know the odd thing or two that is considered common knowledge, it happens to me too. But when it's lots of things, and if the person is wilfully ignorant, that's different.
I once dated a guy who had grown up in the USA, then moved back to the UK in his early 20s (he was British) and I couldn't help but be shocked.
He couldn't name the capital of Italy and other major European countries, couldn't name a single artist (not even someone like Leonardo da Vinci), didn't know anything about the two world wars or other major historical events, thought classical music meant classic rock, had never heard of Labour or the Tories and knew nothing about UK current affairs.
His argument was that he'd grown up in America and at school there they weren't taught much about Europe/the Uk but he knew lots about the USA. Ok, but there had been 5 years since he'd left school and for 3 of those he'd been living in the UK. His parents were also British. Didn't they ever discuss things at home, hadn't he read/watched any news since he'd moved back, listened to the radio, read a magazine?
The answer was no, it didn't interest him but he did know a lot about the science subject that he studied at uni and read a lot about that, plus other science things.
I mean, I get it, we all have different interests and he was very good at his subject... but to not have any curiosity at all about things outside of a few specific interests, including your own heritage and the country you're living in, just baffled me.
Even pop culture references - he'd never heard of David and Victoria Beckham. Not that I'm interested in them, but at the time they were ALL over the media. You'd have had to literally never look at any news outlets to miss them.
I'm afraid it was a turn off for me.