It's not quite that easy. Some marketing content, like blog posts, has long-term value that can't be easily measured in the same fiscal period it was created in. A huge task of marketing is to increase the awareness and value of a brand, getting better search result rankings and helping more people find and trust the company. These effects absolutely affect the bottom line, but they can be pretty hard to measure.

Marketing also generally has multiple touch points before a sale actually occurs. Maybe you see a Google paid result that you ignore, then a YouTube pre-roll that you tune out, and then you see their logo on a train ad during your commute. None of those result in a sale, but you finally see the company at a conference, grab some swag, talk to one of their salespeople, and become a customer. In this example, their revenue looks like it was generated by the efforts of the sales department, but in reality marketing had a lot to do with it, too.

If your company has all the right tracking in place, you can determine the path your customers took when they ultimately bought your product, provided that they bought it online. That still ignores their prior exposure to the product that helped convince them during the current session. More importantly, if you were to measure marketing performance based solely on those metrics, your marketers would have little incentive to do all of their other important functions.