Understanding web accessibility guidelines is like having a map. It gives you the direction, but won't guarantee arrival at the destination. Many organisations fully understand the importance of making their website accessible. Most will even have a roadmap. Too many fail to follow it and implement robust accessibility practices.
Quoting the WCAG isn't sufficient. You need to follow the guidelines and implement the recommendations.
Knowing about ARIA and ARIA patterns won't guarantee you won't screw up the code. You need to continually and manually test the widgets you create to ensure they're actually accessible.
Hiring an accessible consultant won't give you the green light to claim your website is accessible. You need to actually work with them and implement their recommendations.
Outsourcing the work to an outside agency won't prevent future accessibility issues from rearing their ugly head next sprint. You need to do the work.
And that's the point. Only through your sustained efforts and commitment can you ensure a continuously accessible outcome. Having the accessibility roadmap counts for little without actually making the journey.
Having the key and actually opening the door and walking through it yourself are two different things.