0 K is like when there is 0 heat basically, while celsius isn't. Imagine a unit for distance called "goob" where 0 goobs is 100 m and 1 goob is 115 m. In that case the goob unit would behave differently than a meter when you multiply and divide because 0 of the units don't actually correspond to "nothing" in a physical sense. That's exactly how the Celsius scale is, with zero being placed somewhere arbitrarily, not at a physical zero.

Kelvin is just our word for it, but that is the point of “no heat”. It isn’t arbitrary, there is no “negative kelvin” just like you cannot make something colder than absolute zero.

So if you take the difference between “coldest possible temp” and “average summer temp”, then slice it in half, you’re getting temperatures that would kill most life on earth.