Taxing on net specifically reduces integration. Imagine if every business was taxed on 30% if their revenue. You would buy an iPhone for $1000 including $300 tax, and $700 to Apple. Apple pays $700 to Foxconn, who pays $210 tax and gets to use $490 to make the phone. But now imagine Apple buys Foxconn and makes it one company, now they keep $700 to make the phone with. Taxing on net fixes this.

It's not required for personal income because you can't conjoin yourself with your butcher to become one person.