Because the battery -> mechanical energy process is basically lossless while the petrol -> mechanical energy process is lossy in a way that is largely insensitive to the power output.

ie. an EV that needs to produce 1kW might use 1.1kW from the battery and when it needs to produce 2kW it might draw 2.2kW (~2x) from the battery. So 10% increase in weight might result in a 10% reduction in range.

An ICE that needs 1kW mechanical energy will probably burn 10kW worth of petrol; when it needs 2kW of mechanical energy it’ll burn 11kW (<2x) of petrol. So a 10% increase in weight would result in a <10% reduction in range.