Because most chips and peripherals that work at a decently high speed on any computer use DMA. A parallel port on a computer, a thunderbolt connector, Ethernet

Because most chips and peripherals that work at a decently high speed on any computer use DMA. A parallel port on a computer, a thunderbolt connector, Ethernet cards, etc all use DMA for similar desires to speed up communication, and all can have the potential to be attacked. In order for a phone to give decent performance, DMA becomes more and more of a necessity.