That needs a little context, especially since those ships use heavy fuel oil as their fuel and that is becoming increasingly restricted by countries around the world. That reason is part of several responses I found online, to include one that also did some basic math on ships vs cars:"Just one of the world’s larger container ships can emit about as much pollution as 50 million cars. Further, the 15 largest ships in the world emit as much nitrogen oxide and sulphur oxide as the world’s 760 million cars."
"A single RO/RO can carry an average of 8,000 cars, trucks and other equipment. Let’s take an average RO/RO shipping 8,000 cars from Hong Kong to Los Angeles. This is a distance of about 7,286 miles....The RO/RO however only requires 510 Metric Tonnes of fuel to move all of those vehicles and itself from Hong Kong to Los Angeles. This means the ship only used 7.8% of the total fuel required by all those vehicles for the same distance."