Web accessibility is a lot like acting. You need to put yourself in a different reality to do it well. It's not enough to follow a script (or the WCAG guidance), you need to go deeper and understand the why. Why are we making sure images have equivalent text alternatives? Why do we need to know the reading level of the page? Why do we give enough space for things to be readable?
You also need to think of the how. How will our fancy new transition affect someone with vertigo? How will a non-tech-savvy user know where the menu is? How will following the latest visual design trend affect our users?
Being able to put yourself in someone else’s position and situation, either by emulating a condition or by being able to fully understand what it’s like, goes a long way toward making inclusiveness a part of your project before you even begin user testing.
Getting accessibility in at the start means it becomes cheaper, it becomes engrained, and nothing goes out that isn’t built with people in mind rather than ‘users’.