Question

why don't we remember dreams

Answer 1

A Dream is a small perception of things we have discussed in our day.
This happens because the nervous system doesn't allow to consolidate our memory when we wake up.

Answer 2

$\underline{\underline{\tt \: DREAMS \: }}$

Almost all dreams occur during rapid eye movement (REM) sleep. It appears that the content of the dream is stored in short-term memory and cannot be transferred into long-term memory unless they are somehow articulated. Sleep studies show that when individuals who believe they never dream are awakened at various intervals during the night they are in the middle of a dream.