Well ADHD is a condition that affects your ability to do things, it’s called executive dysfunction. It quite literally does prevent you from doing things you need to do. Also, the reminders and stuff work to a certain extent but they rarely do. It’s a problem for almost ADHD people that they don’t work. And yes, I am aware that it’s important.
Mental illnesses and conditions can have extreme physical manifestations and they’re different for each person. Someone may have issues taking a shower, some have issues with brushing teeth. Telling someone to use reminders, while attempting to be helpful, can be considered as patronizing or infantilizing someone. Of course we’ve tried reminders, sticky notes, etc. - it’s that our brains legitimately will not allow us to get up and do something. It isn’t as simple as we want it to be.