Because if the business grows, then you will need to add servers, and reallocate each existing shard's min/max values. If you left room to grow (divide amongst 2^31 instead of 2^63) you can go a long way before you have to solve that problem. If the business turns out to be wildly successful, you'd probably set up a whole new pool anyway and migrate over (and get a new set of pre-allocated ID ranges).
This might be the perfect case to use an UUIDv7 as a key instead of a number.