Because the tax system isn’t designed to be based solely around income tax. In order to make up the missing funds where corporation tax ought to be, personal taxation would have to be considerably higher, which would also mean that employee wages would have to be higher to compensate. The overall perctange of revenue paid into the tax system by the corporation would be the same via higher employee wages.

However, we do not live in a personal taxation only system. Corporations are therefore able to radically reduce their tax spend by avoiding corporation tax and not increasing spend on wages. The total amount of tax paid by the corporation goes down. This is obviously true, otherwise there would exist no motive for companies to pursue strategies of paying such low corporation tax.