Your stack has to grow towards something. Sure, you can have it grow towards the bottom of the address space (which, due to wraparound, is also the top - where

Your stack has to grow towards something. Sure, you can have it grow towards the bottom of the address space (which, due to wraparound, is also the top - where it will safely collide with the kernel addresses) but that only works for one stack - as soon as you create another thread, its thread has to grow towards something else.