To me, it is obvious why sales people are paid by commission. It's because the most efficient thing for a company to do is pay someone their precise marginal value, not more or less, as that leads to the incentive of people doing the best they can given special constraints that only they personally know such as their own personal productivity function and how much they value their own time and effort.

Companies would do similarly for managers and engineers if there were a discreet non-gameable measurement of their output (I'm sure you don't need much imagination to see why paying per JIRA task completed wouldn't work). Unfortunately there's no such measurement system, so the best solution companies have found so far is pay them a salary and try to incentivize effort through bonuses, raises or promotions.