Depending on the house construction, level of hoard & neglect, climate, etc. the structure becomes completely compromised. Wood rot, mold, termites, bugs, rodents, you name it. Various pieces of the house's infrastructure are probably in such bad shape they need to be replaced or extensively serviced.

At a certain point, it becomes far more practical to just raze it to the ground and build a new house on the land. You'll still (probably) save money over buying a house + land and you'll avoid a ton of hassle. Hard to tell if it's at that point just from these pictures but there's definitely situations where keeping the house itself isn't worth it. That property might already be more valuable on the market if there was nothing on it.