Ok, I take your point about gerunds.
But, it's simply a fact that Old English (the language spoke in England before the Norman Conquest) use to have -s ending plurals.
There were two types of plural endings in Old English.
daeg - dagas (day)
nama - naman (name)
Sorry, this is just a fact. The influence of French, especially from 1066 onward, may explain why the -s plural ending became more popular than the -n ending, but it's just a fact that English always had -s plural endings for some nouns.