Adding battery cells is not as efficient for range as adding gas tank capacity is for range. Just like how old cars had severe parasitic power loss from air conditioning systems, power steering systems, and other belt driven accessories, EVs have the same problem but with weight. Adding battery cells can increase range, to a point, but it needs to be balanced with the weight and surface area that is being added to accomodate it.

Using my own '68 Ford as an example, a full tank of gas adds ~170 lbs of weight, with a 27 gallon tank (a gallon of gas weights ~6.3 lbs. I get ~15 mpg, so that's a range of ~405 miles, if I always drive optimally. My Ford's curb weight is 3,765lbs (that's the weight with all fluids filled but no driver or passengers).

Compare this to a Tesla, as AFAIK they have the best range of EVs currently. The Model S battery weighs ~1,200 lbs, which probably makes it the single heaviest component on the entire car including the chassis. It has pretty efficient electric motors at ~95 kWh, so the official EPA rating puts it at a range of ~395 miles, but it can also get up to ~405 miles with smaller wheels and more favorable driving conditions. The Tesla Model S curb weight is ~4,600lb, give or take ~150lbs depending on model year and trim.

~170 lbs compared to ~1,200 lbs is crazy weight for about the same range. Increasing battery density/size is not going to be nearly as efficient as increasing gas tank capacity. And that doesn't consider where the battery is going to go. Bigger battery means bigger footprint, if the car gets bigger that is more surface area, more drag and more weight, and therefore less range. Aero is a good place for EVs to optimize because of this. ICE cars also benefit, but they can afford less efficient aero in favor of aesthetic styling.