And since I didn't complain about it
Well then there's the obvious question why does superhealing with potions ruin the realism for you but not superhealing by sleeping?
I already answered that but let's go around one more time.
KCD is a video game whose chief purpose is to be fun to play. The setting of this video game is a semi historical one, based firmly in a real time and place. The devs made it known that the game was supposed to be relatively close to real life, not high fantasy, no magic or ninjas, etc.
The route of complete and total realism is obviously not suitable for the main goal of the entire medium. It wouldn't be fun to have to rest for 8 months in real life because your character broke his leg.
However, realism is still a goal of the setting. So while a wizard who heals you would work for the medium(video games), it doesn't fit the setting.
A good compromise, and a very common one, is that natural healing is insanely effective and fast. You could say 8 months passed instead of one day, but then the time span of the game gets ruined.
KCD went with this. You can sleep and heal to full.
However, for some reason they also said "Also you can just drink magic potions to heal". This seems contrary to the setting, and doesn't even solve a problem. I think they undercut their own design by including a fantasy trope that doesn't fit the setting, and seems like it was included to ape TES alchemy.