Blight (early blight) spores live in the soil for a long time, but the active organism does not. They won't become active/infective until contacting leaf/stem tissue after splashup or wind blown. They are very resistant to environmental conditions so sterilization methods that would kill active organisms (nematodes, wilts) would be fairly ineffective against spores. You would have to sterilize the whole yard because spores would be there too and blowing into the garden. Yes there can be a huge buildup in fields that are not cleaned of crop residue, i.e. dead plants are plowed under, but that is different than a garden where the plants are removed.
Late Blight, on the other hand, remains active in the living potato tubers, many of which sprout as volunteers the next year, reinfecting potatoes and tomatoes. This is a case where rotating the crops out for a year is necessary.