Because this doesn't guarantee a uniform background.

Let's say we have two teams, the Ayyys and the Bees; each has two defenders, Schlub and Superman play for the Ayys, and Scrub and Slacker play for the Bees. Let's assume that they all suck equally, except for Superman, who is great.

Now we run your statistic: we compare how the Ayyys do with Schlub on/off against how Scrub does on/off.

The problem is, when Schlub is off, Superman is playing, whereas when Scrub is off, Slacker is playing. This means the Ayyys do quite well when Schlub is off, but the Bees do about as well when Scrub is on compared to when Scrub is off. By this comparison, Schlub is a much worse player than Scrub, even though in actual fact they might be equivalent.

ESPN introduced a stat called real plus-minus in 2014, which adjusts for teammates and opponents. It also takes both offensive and defense plus-minus into account. The top two players in RPM this past season were Lebron James and Draymond Green, followed by Chris Paul and Steph Curry [1]. It was received with a lot of skepticism back when it was introduced, but that shouldn't surprise anyone.

1. http://espn.go.com/nba/statistics/rpm/_/sort/RPM

Yeah, that occurred to me but I figured it would average out over enough games. It would be super bad luck to always get subbed in and out in sync with your opponents' best players all season.

They do measure that, and they also measure +/- for a similar purpose.

FYI "+/-": A metric that looks at how teams perform with a certain player on the court, how they perform with a certain player off the court, and calculates the overall impact that player has on team success.