Turns out the answer is complicated. I learned a lot in writing this answer, because I didn't want to just say the same thing everyone else does. I wanted to find out why the answer everyone gives is true.

The energy density of gasoline or diesel is a lot higher than for batteries. This is where the answer starts, and what most people would say.

Let's look at a pair of vehicles that are both tuned for efficiency, a Tesla Model S and a Toyota Corolla.

The Tesla Model S long range has a battery with a life of 103.9 kWh. To convert kWh to kJ, multiply the kWh value by 3600, so the energy in those batteries is 374,040 kJ

Gasoline contains 31,536kJ per litre, and the Corolla has a tank with 45 litres, so a total energy of 1,576,800 kJ. So there's a lot more energy there to begin with. One thing to be aware of is that the EV is using its energy with very high efficiency (80-100%), whereas gasoline or diesel vehicles use their energy with much lower efficiency, converting only between 20 and 40% of the potential energy into kinetic energy.

So with all that, you might think that the two cars are the exact same on grade going up since 80% of 374,000kJ is about the same as 20% of 1.576MJ -- but you'd actually be wrong!

See, a Toyota Corolla with a full fuel tank weighs about 3014lbs, and a tesla (with a full battery, not that the charge state of the battery matters ) weighs 5761lbs. Some of that weight is because the Model S is a vehicle that costs twice as much as the Corolla and so has a lot more creature comforts that cost weight, but some of it is due to the weight required for large electric motors and a large amount of it -- about 1400lbs -- is the battery. The energy it takes to move something goes up linearly with weight, so during acceleration the tesla will require almost twice as much energy to get up to speed, and just as importantly it'll take almost twice as much energy to raise the car on a hill.

And then the energy density thing becomes worse as you go. If you want to double the energy in the Model S, you'd need to add another 1400lbs of battery, but if you want to double the energy in the Corolla, you add another 45 litres of gasoline weighing about 80lbs.

This problem gets much bigger if you're not driving a small passenger car. An F150 v6 gas weighs 4564lbs and carries 87 litres of fuel, so about twice as much energy as our Toyota Corolla. An F150 Lightning weighs 6500lbs, and has a battery capacity slightly less than the Tesla we just mentioned for the standard capacity model, or slightly more (131kWh) than a tesla for the extended range model. Given everything, scaling up EVs ends up becoming less capable because you need to add so much weight to add some range.