Long story short, because the 25 year old thermal plant runs near peak efficiency most of the time, whereas an ICE powertrain is inherently inefficient even with recent advancement. It's ironic that the most  efficient ICE systems are those that are actually electric, and charge off a sporadically operating high efficiency engine, isn't it?

For the sake of comparison, though, a Chevy Sonic burns about 8l/100km, emitting circa 18kg of carbon dioxide. A Bolt uses about 18kwh/100km, and a coal fired thermal plant emits about 0.9kg of carbon dioxide per kwh,.. meaning that in the worst case scenario, the two emit comparably.  In the Canadian context, where actual emissions per kwh range from 0.6kg to virtually none, the electric wins every time.