Why do we say "assistant" instead of "assister"?

Is it just arbitrary?

A Google search gives me the etymology of "assist" but not the reason for the convention. It also notes that "assistor" is the spelling used in legal documents.

accountant/accounter
defendant/defender
commandant/commander

Looking at the list above, is it when it's an official job? For example, if I help an elderly person cross the road, I am an assister but not an assistant? If I calculate the combined total of my bank accounts, I am the accounter but not an accountant?

(from the comment)

English "assistant" was not formed in English from "assist" plus "-ant".

Instead, etymology shows "assistant" is from Medieval Latin "assistent". The "-ant" or "-ent" ending shows a present participle in Latin.

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