Washing Polyester fabric does soften it some when you wash it.
Polyester does get softer sometimes when you wash it but not really soft.
If you really want to soften the Polyester fabric you should use some fabric softener in the wash when you wash the polyester.
Another way to soften polyester in the washing machine is to use some vinegar to soften the polyester.
Put the polyester in the washing machine and and let the washer complete it's wash cycles.
After that is done pour 1 cup of undiluted white vinegar into the washing machine during the final rinsing cycle to soften the polyester.
The vinegar is much better for the polyester to soften it because the vinegar does not infuse the polyester fabric with chemicals.