Why can't you refreeze food?
You can refreeze food but it depends how you handled it before that
Once it's been defrosted the temperature of the food can enter the food temperature danger zone (5 - 60 degrees), where bacterial growth is rapid and the risk of food poisoning increases
This is an issue if you defrost food in the microwave or out of the fridge. If you leave it in the fridge to defrost it, you could refreeze
If you defrost food safely (eg. in the fridge, then cook it and put in the freezer again as soon as it's cool or just straight back to the freezer) it could be safe to eat once thawed again but the quality of the food would be reduced ie. the freezing process could make it mushy
Also some foods are just bad when refrozen because of the textural changes