yeah, I think that's the misconception...
these are my thoughts
1. 63, while very young (too young to die!) and certainly everyone said BA was healthy and physically active - it still is a lot to take care of another person (Gene) and cook & clean a 9,000 square foot houses.
2. Having 3 active dogs going in and out of your home in the country may lead to some rodents getting in as well.
3. Rodents will find a way

They need a very small hole to get into your house.
4. Even though BA sounds healthy and active, she may have made caretaking GH her primary goal. If GH was very high needs (as he sounds he was) with advanced dementia, she could have procrastinated getting rodent "pathways" filled in.
overall, I think - even though they are very wealthy - rodents just need a small pathway into your home.