Personally I don't buy that sales skills are so much more difficult to find or are in higher demand than software development skills. Some of the difference in equity certainly has to do with the higher risk of getting fired for poor performance in sales.
But beyond that it's probably that sales and business people are motivated mostly or only by money and thus have spent decades as a collective optimizing getting more money for themselves. In contrast, developers and designers are motivated by accomplishing and creating good things before money and end up not focussing on optimizing money making.
If developers were compensated like lawyers, I doubt they'd be so worked up over salespeople getting commissions.