This is a mental illness or something wrong. Either they grew up with hoarders or something happpened to them to cause them extreme insecurity and they start hoarding, finding security in stuff around them.
They are not dirty people. My firend showers and washes her hair every day. Her clothes are always clean. In fact they are almost alwyas new becuase hse can't find most of her clothes so she has to keep buying them.
She would help me cook sometimes and washed her hands before she handled dishes or food, her hygenic habits are fine, most hoarders are that way.
She just has tons and tons of stuff that make it impossible for her house to be clean - you can't have 10 years worth of newspapers and magazines piled up and not have some issues with insects or rodents. But in her mind she may need to refer to an article in one of those papers from 7 or 8 years back. She can't grasp the concept that she could never find the article.
She literally has thousands of kintting and crocheting books. To find a crochet pattern she saw a few months back would be impossible.
Like Linda said, they really believe they are helping when they have all the animals. My friend has no pets, had a cat at one time, but not now. She does want a cat, but just one.