Not everyone has their carpets cleaned professionally, especially if they live in a rental. The problem is that while a hard floor surface shows up the dirt and dust on it, a carpet does not. At least, it is not nearly as visible. The carpet is also likely to become uniformly dirty so you don’t even notice it until something like the sofa that has been standing on the carpet for a long time, is removed. You can then see a lighter patch of carpet, or a patch where the colours are much more vivid. Only then will you realise just how grubby the rest of it is.
Pet hair and dander
Dirty carpets can certainly become a health hazard, though you may not even realise it. If you have pets inside, their hair and dander will become embedded into the carpet and can cause an allergic reaction, especially to people like children, who get down on the carpet where they are close to the allergens. Even dust and dust mites can be in the carpet and cause allergies to flare up.