Because (1) a tank full of petrol can easily be 85% petrol by weight, (2) when it burns in your ICE, over 75% of the resulting weight (CO2 and H2O) is "free for the taking, everywhere you go" atmospheric O2, and (3) you need to keep carrying 0% of that exhaust weight with you.
With batteries, all those percentage go to crap. Plus you've got the extra structure needed to carry all the extra weight, all the time.