That's how the engineers set it up. No one has yet been able to observe any degradation on Ioniqs or Niros yet. 2016 was the first model year (HEV) and some have well over 100,000 miles. Obviously, it is not possible to have zero degradation. It will be easier to spot on the PHEV eventually, but the BMS is designed for very long battery life (which the reserve capacity helps tremendously - you can't fill the battery to 100% which degrades lithium batteries faster). If you think about warranty issues, obviously it is better for KIA to not have consumer observable degradation before 100,000 miles.
2018 Kia LX HEV Metal Stream with Advanced Tech