Economies of scale. Generating power - whether it's fossil fuels, wind turbines, hydro, whatever - is more efficient at scale. To put it another way, a single 1MW generator will use far less diesel than a thousand 1KW generators. Also, electric cars are insanely efficient compared to combustion engine cars, so even if all your electricity is generated in diesel power stations it's more efficient than burning it inside the car. Additionally, large centralised power stations are better maintained, not constrained by weight and can have offsetting/capture systems attached that are impossible in a car that must, above all else, be small and light enough to move.

Renewables and nuclear are still the way and a carbon-heavy grid makes it take longer for EVs to break even but even then EVs are a no-brainer.