Because sales are quantifiable and directly mapped to performance.

To get that kind of proportional payback in engineering you'd need very clear financial objectives for a project. I could see that happening in optimization scenarios where consultants are brought in and get paid for whatever they can trim from operational costs.

In fact I’ve seen both tech and manufacturing efficiency consultants whose quotes include $x up front plus monitoring and reporting that shows the efficiency gains. Then rather than taking a closing fixed payment, they get a percentage of the savings to the client over the first six or twelve months.