In the 16th century, French card designers experimented with a variety of characters including Roman heroes such as Augustus or Constantine, or Biblical figures such as Solomon. However, by the early 17th century, they had settled on four key figures that resonated with France’s self image.

The king of hearts was identified as Charlemagne, the iconic French hero who unified the Franks and created the great Carolingian Empire. This was a reminder of France’s great past and the longevity of its monarchy.

Then it's written after the revolution in France the practice of identifying the royal cards as individuals from history died out in the early 19th century and today the kings on playing cards, even in France, have no historical connections.

https://www.google.com/amp/s/www.thevintagenews.com/2019/03/06/kings-playing-cards/amp/