The short answer is that some kids still need them even as late as age eight or nine (even during the day).
Due to a neurological condition that would only be diagnosed much later I was a late trainer myself. I was five-and-a-half, almost six by the time I was pretty much there during the day. I was at least nine or ten before my nighttime wetting began to drop off in any significant fashion. I may have been as old as eleven (or close to eleven) by the time it stopped completely.
Back in the 1980s, one of my cousins didn’t start training until between age four and five and his bowel training wasn’t finished (even during the day) until he was seven years old (in second grade). He had to wear diapers at night for at least another year beyond that. His nighttime training finished at an earlier age than mine did, but for some reason he got a later start. He was normal sized (or even slightly larger than normal) so his parents needed to resort to cloth diapers and plastic pants by the time he was about six or seven (before that he was able to wear Pampers). I was able to stay in Pampers for as long as I needed them because I was smaller than average and by the time I had my growth spurt in late childhood I no longer needed them. All of this happened during the 1980s.
The bottom line is that these products exist in this size because there is enough of a market to make them commercially viable for the manufacturers. Also, some children as young as four or five who may still need to wear diapers either occasionally (at night or in other situations where it may not be possible to get to the bathroom in time) or all the time are overweight or obese (for various reasons) and therefore need these products in the larger sizes.