Nobody is selling "Falcon X rockets". Space X sells rocket launches, because that is what other people want to buy. And JK Rowling could most definitely afford a couple rocket launches if she were so inclined.
The Model T was released over a decade prior to the point mentioned in the article. Sure cars weren't the staple of American life they are today, but they were definitely accessible to the well-off people of the time.
They also weren’t as common - and so not as necessary.
Those who wanted to could get one but it wasn’t considered a necessity when there were so many other options - including sending a servant to the market for your servants, so no need to drive yourself.
The article mentioned that the Christies lived in a 4th-floor walkup in London.
When I first moved to the big city, I quickly discovered that the cost of parking and insuring my car was more than the cost of the car. This scenario was most likely worse back in those days as the infrastructure was designed to “store” horses, not cars.
JK Rowling has hundreds of millions of dollars if you want to talk about comparisons that might not be exactly apt.
The difficulty of comparison is discussed in the article, but Agatha Christie and her husband likely had an income that is reasonably comparable to many people today; the maid and nanny were dirt poor...
You might ask why can J.K.Rowling afford a maid and a nanny but not a Falcon X rocket today. Perhaps her great grand children will.