They buy 3 since virtually all can convert between two levels (any two adjacent levels). A rare (and expensive and heavy) few can do 3 levels in one.

But converting the seat requires rethreading the straps and other adjustments - it can take an hour to install some types if you are not familiar with it.

I'm not sure how it applies to other countries, but here at least you have to have special car seats for all children until they're aged 12. It's insane, but that's the fabulous new law they enacted. So, yeah in some cases, some parents really are buying that many seats.