in my experience, people with large incomes often have more rooms in their house than they actually need for day-to-day life. sometimes it's just a flex, but often they expect to host their extended family and/or friends (and even their families) over the holidays. nobody needs a second kitchen and four extra bedrooms, but it does make bringing the whole family together under one roof a lot less tense.
It's really not hard to find use for rooms.
One is an exercise room. It's the first photo here [1].
The one in the second photo is the electronics and music room. Table on the left for my electronics stuff, and piano on the right (and guitars out of frame on the right, along with a bookshelf full of music books).
Third bedroom is the library, covered by the next two photos. 10 bookshelves.
I could easily see two developers use 6 bedrooms. 1 for sleeping, 1 for a library, 1 for exercise, 1 for a join hobby for interest, and 1 for each for a hobby or interest that the other doesn't share...and that's without even considering using a bedroom as a dedicated office for working from home.
Some people like feeling of living in a palace. We have after all for millennia equated living in huge spaces with class and prosperity, and The industrial age, especially in last 30 years, has enabled more people than ever to live in palaces.
Of course, heating a six bedroom palace in Minnesota is no joke.