[Edited to add more ”meat” to the comment.]
The fact that he also owns several sports cars and a villa on the French Riviera, is not something he wants publicized.
Steve Jobs never swanked up his house either.
Bill Gates's mansion was built primarily to try to showcase Microsoft development efforts (don't laugh! okay, laugh).
That seems like a bad idea to me. Old Volvos were indeed very safe by the standards of the time, but compared to a modern car, they're simply not. Here in the US, 30,000 people die in auto accidents every year (it's somewhere around 250k/year worldwide). If you have a ton of money, you might as well get a new car and protect yourself. A brand-new bottom-of-the-line Volvo will let you walk away from crashes that some old Volvo would kill you in. And surely a new V60 or S60 (their lowest-end models IIRC, unless they still have S40s there) wouldn't look too out-of-place in Sweden of all places (where the cars come from).
I would rather someone who has legitimately made a billion dollars just go buy a yacht and a Ferrari and drive them around.
Kamprad has a pretty solid tax-avoidance scheme going on too. Completely legal but frowned upon by those who think he should "pay his fair share".
But he is the exception not the rule when it comes to how the wealthy live typically.