The thing is that ICE vehicles are so inefficient to begin with, but a towed trailer needs the same amount of energy regardless of what is towing it.

Imagine a trailer takes 1 unit of energy to move, and ICE truck takes 5 units to move itself, and EV truck takes 1 unit to move itself.

The ICE truck towing it would go from 5 units to 6 units, reducing the range by 16%.

The EV truck would now go from 1 unit to 2 units, reducing the range by 50%.

So the better your efficiency, the worse you are at towing. Ideally EV trucks would have such a huge battery that even when the range is reduced by 50%, it is still as long as a gas truck's range, but modern battery technology is still too sucky to achieve this.