why is good grooming so difficult??

Many of us post here about lack of grooming in our ADHD husbands.  Why is grooming such a big issue with them?  My DH has the most disgusting feet and toenails you can even imagine.  Why can he not keep them trimmed??  He walks around barefoot, if I can see them he can see them.  I have asked him to keep socks or slippers on because his toes are gross and he tells me to get over it, it's just feet.  Ditto the fingernails--long, hangnails, raggedly.  Why??  Stuff in the ears--yuck.  Why is keeping yourself together such a big thing?  He showers every night and he does brush his teeth but his breath is terrible even still.  I have no idea why.  I am not being sarcastic--I want someone to explain the connection between ADHD and poor grooming.  I wonder if it comes from the same place as the "not seeing the mess" comes from--like those of us with husbands that can walk right by an overflowing garbage can and literally not see it.  That kind of thing--a lot of us post about that too--how do they NOT see _____________ (whatever it is).  Maybe it's the same with grooming.  I refuse to tell my 48 year old husband his nails need trimming.  My two teenage boys don't need reminding for crying out loud.  Don't you care what you look like?