Simple answer: because it was made that way.
More complicated answer: to keep the lens fronts have interchangable filter sizes with other lenses. And 67mm is a popular size.
Actual answer: zooms like the one you mention are made for SLRs mainly, so maybe yours is for an SLR, I do not know. True tele lenses with fixed focal length are shorter than their focal length would indicate, hence on RF cameras one can get away with a smaller opening for the front lens. And on SLRs, the tele lenses and zooms are often retrofocus designs and need more extension to clear the mirror, hence wider front elements are used than the ratio of focal length to max aperture demands.
I would stick with the first answer if you are happy with it.
