> How come it works for basically every other job on this planet?

In sales it is uniquely easy to identify your contribution since it's literally measured in dollars coming in.

There's no way to quantify that for feature implemented/bug fixed, it would devolve into endless politics.

> how come sales people are unable to do things for a fixed monthly salary?

Sure they are able, but no good sales person is interested. Presumably you'd want to hire the good ones. You can certainly staff a mediocre sales team on a fixed salary, they just won't do much selling.

> In sales it is uniquely easy to identify your contribution since it's literally measured in dollars coming in.

This logic about stops working as soon as you sell something that is not some immediately exchanged mass produced commoditized gadget. Sales people on commission have tons of incentive to saddle the company with demands and imaginary product features they can't actually deliver on and be long gone before the bill comes due.