I'm not going to use the word "Nasty" cause I find it a bit rude and insulting. I think that everyone has different priorities and different levels of tolerance for some things. Obviously you have a low tolerance for clutter, dirt, grim, etc.. while others have a high tolerance.
Let me give you an example you have two roommates and one is always getting upset that the other doesn't do their fair share of choirs around the house. The problem is that the second one doesn't have the same idea of what is clean vs dirty. If you don't think something is dirty why would you clean it.
I work with a lady with OCD and I find it interesting that she can be very compulsively clean about some areas of her life but ignore other aspects. Such as there are piles of boxes, tissues, and other scraps all over the floor and cabinets but she will focus on the fact that one drawer isn't pushed in all the way or she will want to dust just one object even though the object is in a pile of junk. We have a great ability to tune out anything that isn't important to us and selectively bring in information. For some people clutter and cleanliness are not important and are just tuned out. My brother is a very unclean person by nature and if left to his own devices he would live in piles of dirty clothes, dirty dishes, etc.. His excuses are he doesn't like cleaning, isn't good at cleaning and it doesn't bother him.
Some reasons why people might live an unclean lifestyle could be due to limited education about sanitary practices or cleaning methods, trauma (shifts focus away from cleaning), not of importance, physical disorders that limit a persons abiliy to clean, Mental disorders that may shift the focus away from cleanliness (depression, OCD, Autism, etc..), large family (many pets) household where it is difficult to keep up with messes, don't have time/money, inherited mess, etc..
I don't think there is any fix all for these people that live unsanitary lives. People have to want to change in order to change. Someone who hoards will still hoard even if their house was thoroughly cleaned and all the extra items gotten rid of. Making them live a clean lifestyle would be the equivalent to making you live a dirty lifestyle, it just isn't going to happen.