How do they know it does? Perhaps we only perceive it flowing in one direction only because we're in an accelerated frame of reference with respect to time.
An "accelerated frame of reference"? LOL.
We "perceive" time as flowing forward because that is the nature of time, the reality we live in.
God created time, days and seasons.
We perceive time not only in our personal frame of reference but on EVERYTHING outside of of ourselves as well. Everything has a shelf-life, gets old, a period of reckoning and we SEE this. That is the definition of time.