I'm wondering why commissions are structured the way they are. For example, in a SaaS company with a subscription model, I'd imagine that it would align motives

I'm wondering why commissions are structured the way they are. For example, in a SaaS company with a subscription model, I'd imagine that it would align motives of sales people better if the commission would be paid from each monthly payment instead of as a lump sum when the deal is closed.