Because it’s not a point and shoot. Such motorized lenses are fine for simple tiny consumer-oriented cameras, but SLRs with interchangeable lenses are meant for a different market.

All Canon EF zoom lenses but one are manual zooms. That is to say you adjust the focal length either by turning a ring (two touch) or sliding the lens in and out (push-pull). And most people find that adjusting the zoom setting on such lenses is much faster and more precise than a slow and cumbersome motorized point and shoot zoom lens.

Note that Canon did briefly sell a motorized zoom lens for EOS cameras a while back, presumably as a sort of experiment. That lens, the Canon EF 35-80 4-5.6 PZ (Power Zoom), was an all-plastic cheapie with fairly low optical quality. The lens barrel had two pushbuttons that let you adjust the zoom position.