Canon 24-70 2.8 II doesn't have IS and it's a stable in most wedding photographer lens arsenal including me. How did all these wedding photographers including me survive before IS?
No other camera manufacture have created something ground breaking in this zoom range as 28-70 F2. Not Nikon, Sigma, Zeiss, Sony, Tamron, Batis etc.
28-70 is already 3lb and some already complain about the weight which is similiar to Sigma 105 1.4 Art/70-200 2.8 II IS. They would have added an extra 4mm and make 24-70 F2 IS if it's an easy engineering feat.
Engineers have to make compromises on cost, size, weight, and optic performance and under marketing team decision.