We need to avoid generalisations: yes, there is no definite rule on shoes in the house in Italy, but that does not mean that all Italians always wear their shoes inside the house, only that there is no rule and each of us has the choice.
In fact, most Italians take off their shoes and wear slippers or socks or nothing inside their house, but it is not a strict social rule. So, you may find someone who wears shoes in their own house, and many who do not.
I believe that there is also a misperception of this issue, due to the fact that foreigners are usually guests in Italian houses and they are the ones asking those kind of questions on Italians’ habits. Actually, I am used to wear slippers inside my house, but I have been taught by my parents that it is not polite to be in slippers when your guest is wearing (potentially uncomfortable) shoes in your house, as it may be unfair to your guest: so, when I have someone calling to my place I put on shoes, so as to be on the same footing as my guest (sorry for the pun). Of course, from the guest’s point of view, I have always been wearing shoes inside my house, whereas in fact I have put them on only moments before opening my door to him/her and I am longing to go back to my normal slippers. It used to be deemed impolite to ask your guest to take off shoes when entering your house, as you should not make your guest embarassed or uncomfortable by imposing your rules on him/her, but trying to make his/her stay the more agreeable for him/her as possible.
In recent times, I am offering slippers or at least the choice between wearing shoes or not to my closest friends and family, but only a few years ago my aging parents would have thought it impolite and vulgar, like saying to my guest that they had to because their shoes were dirty or something like that.
In the very popular very old children novel “Cuore”, one of the children characters is chided by his father for brushing away dirt from a chair used by one of his poorest friends: his father explained to him that there should be no shame for a guest for having soiled the chair, because one should look past social class differences and see every good friend as equal, and not embarass him by pointing out his poverty, that was out of his power to change. Yes, I know that this is…not really how it goes in real life, all this goid and open heart towards poor people and working class kids, but the title of the novel means “Heart” in Italian! This novel has influenced many elder Italians: just to let you know where we stand.
So, when a guest enters our house with shoes, we kind of feel that we need to put on shoes as well, so as not to embarass him/her. Afterwards, when the guest is gone, we will likely clean the floors of the whole house.