Same ways gay people get 'straight married'.

Could be family pressure. Could be internalized hetreonormativity making them feel like they 'should' do this. Could be they haven't really realized, come to terms with, or accepted their own identity.

I mean, think of a 'stereotypical' aromantic guy. He's interested in women, and sleeps around a lot, but despite not getting feelings, might 'settle down' with one partner because its 'normal, respectable', even if it's not something that makes him happy. Probably won't make the wife happy either, but that's it's own issue, why she might marry a guy that 'doesn't do romance'.

Mainly social expectations and lack of awareness of aromanticism. I know in the US that's common in the deep south (where I'm from), but I'm sure you'll find it anywhere that's socially conservative.