I've seen accessibility ignored by product teams for two reasons.
They ignored the issue. They deliberately turned away from acknowledging something was wrong and they could do something about it.
Ignorance is comfortable and easy, no matter how you justify it (lack of budget, no training, no time).
You use ignorance as a way to maintain your existing beliefs without the discomfort of challenge or growth. We have no users with disabilities. No one uses screen readers in our product. Assistive technology users are a tiny minority anyhow. Accessible design looks ugly.
You can choose to keep believing that. By definition, ignorance is a choice.
Not knowing is fundamentally different. Unlike ignorance, a lack of knowledge carries no moral weight. It's not a choice you make. It's only the natural starting point for learning, should you want to.
The line between ignorance and not knowing is razor-thin. Once you realise you know, ignoring it is no longer the right thing to do. It becomes a moral failure and your only duty is to act.