Why is my front setback measured from the middle of the road in Macomb Township?

Because the district tables are written that way. Every district article sets front and street-side setbacks from the center line of the road, using the planned right-of-way widths in the Macomb Township Master Thoroughfare Plan (for example Sec. 10.0704.D.1 in R-1). The technique reserves land for future road widening: on a 60 foot local street the R-1 setback is 55 feet from centerline, which after the 30 foot half right-of-way behaves like a 25 foot yard, but on a road planned at 120 or 150 feet the same house sits much farther back.

Practical consequences: your real front yard depth depends on how the Thoroughfare Plan classifies your road, not just your district; M-59 (Hall Road) and Gratiot Avenue have their own setbacks measured from the MDOT right-of-way line instead; and in residential districts the front setback also may not be less than the average of the existing developed front yards within 100 feet on your block face, except on major roads (Sec. 10.0311.C). Before siting an addition or a porch, have the Planning Department confirm your road's planned right-of-way and the resulting building line.