I think we’re all in agreement that soap is not a food. “Not a food” isn’t equivalent to “absolutely impossible to eat by anyone-human or animal.”
Getting back to the original question, there seem to be a number of answers:
1. Way back when, some Jews used laundry detergent for washing dishes because there was no such thing as dish detergent.
2. Some people hold that there’s a problem with using treif laundry products on tablecloths, dish towels, and napkins.
3. Some people regard hashgachas as symbols of quality or purity.
4. The kashrus agencies are willing to accept a fee even if they hold the product doesn’t need hashgacha.