Because everyone has a different core temperature. So it really can't be set value like 100°F.

Generally speaking, a fever of 100°F is "fine", albeit completely exhausting and not at all fun, but it's good for helping the body to fight infections.

Fevers higher than 102°F are when you need to start working on bringing that fever down. If it climbs higher than that, you risk brain injury. The proteins in your body will also start to denature (lose their ability to function) as your temperature climbs, including proteins that work within your brain and your nervous system.

If it continues to climb or won't go down even after taking medicine, you need to call your doctor or go to the hospital.